PART THREE
第三部分
FROM FOOD TO GUNS, GERMS, AND STEEL
从粮食到枪炮、病菌与钢铁
CHAPTER 11
第十一章
LETHAL GIFT OF LIVESTOCK
牲畜的致命礼物
WE HAVE NOW TRACED HOW FOOD PRODUCTION AROSE in a few centers, and how it spread at unequal rates from there to other areas. Those geographic differences constitute important ultimate answers to Yali's question about why different peoples ended up with disparate degrees of power and affluence. However, food production itself is not a proximate cause. In a one-on-one fight, a naked farmer would have no advantage over a naked hunter-gatherer.
现在，我们已经考察了粮食生产是怎样在几个中心出现的，以及粮食生产是怎样以不同的速度从那里向其他地区传播的。这些地理上的差异就是对耶利的问题的最后的重要回答，而耶利的问题是：不同的民族最后何以在权力和富裕方面大相径庭。然而，粮食生产本身并不是直接的原因。在一对一的战斗中，赤手空拳的农民可能不是赤手空拳的狩猎采集者的对手。
Instead, one part of the explanation for farmer power lies in the much denser populations that food production could support: ten naked farmers certainly would have an advantage over one naked hunter-gatherer in a fight. The other part is that neither farmers nor hunter-gatherers are naked, at least not figuratively. Farmers tend to breathe out nastier germs, to own better weapons and armor, to own more-powerful technology in general, and to live under centralized governments with literate elites better able to wage wars of conquest. Hence the next four chapters will explore how the ultimate cause of food production led to the proximate causes of germs, literacy, technology, and centralized government.
对于农民的力量的一部分解释，在于粮食生产所能养活的稠密得多的人口：10个赤手空拳的农民在战斗中肯定能胜过一个赤手空拳的狩猎采集者。另一部分解释是：无论是农民还是狩猎采集者都不是赤手空拳的，至少不是在比喻的意义上。农民往往会呼出更可怕的病菌，拥有更好的武器和盔甲，掌握一般说来更有效的技术，并且生活在集中统一的政府之下，而这种政府里有更懂得如何去发动征服战争的有文化修养的杰出人物。因此，下面的4章将探讨一下粮食生产这个终极原因是如何导致病菌、文化修养、技术和集中统一的政府这些直接原因的。
The links connecting livestock and crops to germs were unforgettably illustrated for me by a hospital case about which I learned through a physician friend. When my friend was an inexperienced young doctor, he was called into a hospital room to deal with a married couple stressed-out by a mysterious illness. It did not help that the couple was also having difficulty communicating with each other, and with my friend. The husband was a small, timid man, sick with pneumonia caused by an unidentified microbe, and with only limited command of the English language. Acting as translator was his beautiful wife, worried about her husband's condition and frightened by the unfamiliar hospital environment. My friend was also stressed-out from a long week of hospital work, and from trying to figure out what unusual risk factors might have brought on the strange illness. The stress caused my friend to forget everything he had been taught about patient confidentiality: he committed the awful blunder of requesting the woman to ask her husband whether he'd had any sexual experiences that could have caused the infection.
一个医院的病例令人难忘地向我说明了牲畜、作物与病菌之间的关系，这个病例是我从一个医生朋友那里听说的。当我的朋友还是一个初出茅庐的年轻医生时，他被叫进医院的一个房间去给一对受到一种怪病折磨的夫妇看病。这对夫妇彼此沟通有困难，同我的朋友沟通也有困难，这对治病是不利的。做丈夫的是一个胆怯的瘦小男子，不知是什么病菌使他得了肺炎。他只能说几句英语，充当翻译的是他的美丽的妻子。她为她丈夫的病忧心忡忡，并对陌生的医院环境感到害怕。我的朋友在医院里工作了整整一个星期，想弄清楚到底是什么异乎寻常的危险因素引起了这种怪病，这使他感到疲惫不堪。身心劳累使他忘记了关于病人秘密的所有教导：他犯了一个可怕的错误，竟要求那个做妻子的去问她的丈夫他是否有过可以引起这种感染的性经历。
As the doctor watched, the husband turned red, pulled himself together so that he seemed even smaller, tried to disappear under his bedsheets, and stammered out words in a barely audible voice. His wife suddenly screamed in rage and drew herself up to tower over him. Before the doctor could stop her, she grabbed a heavy metal bottle, slammed it with full force onto her husband's head, and stormed out of the room. It took a while for the doctor to revive her husband and even longer to elicit, through the man's broken English, what he'd said that so enraged his wife. The answer slowly emerged: he had confessed to repeated intercourse with sheep on a recent visit to the family farm; perhaps that was how he had contracted the mysterious microbe.
这位医生注意到，那个做丈夫的变得面红耳赤，把身体蜷缩起来，使原本瘦小的身躯似乎变得更小了。他恨不得一头钻到床单下面去，用一种几乎听不见的声音结结巴巴地说出几句话来。他的妻子突然怒叫一声，站直了身子，居高临下的对着他。医生还没有来得及阻拦，她已一把抓起了一只很沉的金属瓶，用尽全力向她丈夫的脑袋砸去，接着怒气冲冲地跑出了房间。医生花了一番工夫才把他弄醒过来，甚至花了更大的工夫才从那男人的结结巴巴的英语中探听出他究竟说了些什么竟使他的妻子如此暴怒。答案慢慢地出来了：原来他刚才承认不久前到家里的农场去时和母羊性交过几次，也许这就是使他传染上那神秘的病菌的原因。
This incident sounds bizarrely one-of-a-kind and of no possible broader significance. In fact, it illustrates an enormous subject of great importance: human diseases of animal origins. Very few of us love sheep in the carnal sense that this patient did. But most of us platonically love our pet animals, such as our dogs and cats. As a society, we certainly appear to have an inordinate fondness for sheep and other livestock, to judge from the vast numbers of them that we keep. For example, at the time of a recent census, Australia's 17,085,400 people thought so highly of sheep that they kept 161,600,000 of them.
这件事听起来有点荒诞不经，也不可能有什么更普遍的意义。但事实上，它说明了一个非常重要的大问题：源自动物的人类疾病。我们爱羊很少会像那病人一样是出于肉欲。但我们大多数人对我们的猫、狗之类的宠物有一种柏拉图式的爱。从我们所饲养的羊和其他牲畜的庞大数目来看，我们的社会毫无疑问对它们似乎有一种过度的喜爱。例如，最近的一次人口调查表明，澳大利亚人对他们的羊非常重视，1708.54万人竟养了1.616亿只羊。
Some of us adults, and even more of our children, pick up infectious diseases from our pets. Usually they remain no more than a nuisance, but a few have evolved into something far more serious. The major killers of humanity throughout our recent history—smallpox, flu, tuberculosis, malaria, plague, measles, and cholera—are infectious diseases that evolved from diseases of animals, even though most of the microbes responsible for our own epidemic illnesses are paradoxically now almost confined to humans. Because diseases have been the biggest killers of people, they have also been decisive shapers of history. Until World War II, more victims of war died of war-borne microbes than of battle wounds. All those military histories glorifying great generals oversimplify the ego-deflating truth: the winners of past wars were not always the armies with the best generals and weapons, but were often merely those bearing the nastiest germs to transmit to their enemies.
我们有些成年人会从我们的宠物那里得到传染病，而儿童得这种传染病的甚至更多。通常，这种病只不过是一种小小的不舒服，但有些也会发展成为大病。整个近代史上人类的主要杀手是天花、流行性感冒、肺结核、疟疾、瘟疫、麻疹和霍乱，它们都是从动物的疾病演化而来的传染病，虽然奇怪的是引起我们人类的流行疾病的大多数病菌如今几乎只局限于在人类中流行。第二次世界大战前，战争受害者死于战争引起的疾病的比死于战斗创伤的要多。所有那些为伟大的将军们歌功颂德的军事史对一个令人泄气的事实只是轻描淡写一笔带过，这个事实就是：过去战争中的胜利者并不总是那些拥有最优秀的将军和最精良的武器的军队，而常常不过是那些携带有可以传染给敌人的最可怕的病菌。
The grimmest examples of germs' role in history come from the European conquest of the Americas that began with Columbus's voyage of 1492. Numerous as were the Native American victims of the murderous Spanish conquistadores, they were far outnumbered by the victims of murderous Spanish microbes. Why was the exchange of nasty germs between the Americas and Europe so unequal Why didn't Native American diseases instead decimate the Spanish invaders, spread back to Europe, and wipe out 95 percent of Europe's population Similar questions arise for the decimation of many other native peoples by Eurasian germs, as well as for the decimation of would-be European conquistadores in the tropics of Africa and Asia.
关于病菌的历史作用的最令人生畏的例子，来自随同哥伦布1492年的航行而开始的欧洲人对美洲的征服。虽然被那些杀人不眨眼的西班牙征服者杀死的印第安人不计其数，但凶恶的西班牙病菌杀死的印第安人却要多得多。为什么在欧洲和美洲之间这种可怕的病菌的交流是这样不对等？为什么印第安人的疾病没有大批杀死西班牙入侵者，并传回欧洲，消灭掉欧洲95%的人口？不但在大批杀死想要成为非洲和亚洲热带地区的征服者的欧洲人方面，而且在欧亚大陆的病菌大批杀死其他许多土著方面，都出现了类似的问题。
Thus, questions of the animal origins of human disease lie behind the broadest pattern of human history, and behind some of the most important issues in human health today. (Think of AIDS, an explosively spreading human disease that appears to have evolved from a virus resident in wild African monkeys.) This chapter will begin by considering what a “disease” is, and why some microbes have evolved so as to “make us sick,” whereas most other species of living things don't make us sick. We'll examine why many of our most familiar infectious diseases run in epidemics, such as our current AIDS epidemic and the Black Death (bubonic plague) epidemics of the Middle Ages. We'll then consider how the ancestors of microbes now confined to us transferred themselves from their original animal hosts. Finally, we'll see how insight into the animal origins of our infectious diseases helps explain the momentous, almost one-way exchange of germs between Europeans and Native Americans.
因此，人类疾病源自动物这一问题是构成人类历史最广泛模式的潜在原因，也是构成今天人类健康的某些最重要问题的潜在原因。(请想一想艾滋病吧，那是一种传播速度非常快的人类疾病，似乎是从非洲野猴体内一种病毒演化而来。)本章一开始将要考虑什么是“疾病”，为什么有些病菌演化的目的是“使我们生病”，而其他大多数生物却不会使我们生病。我们将考察一下，为什么我们最熟悉的传染病中有许多成为流行病而迅速传播，如当前艾滋病的流行和中世纪黑死病(腺鼠疫)的流行。然后，我们还将考虑现在只在我们中间传播的那些病菌的祖先，是怎样从它们原来的宿主动物身上转移到我们身上来的。最后，我们还要看一看，对我们的传染疾病源自动物的深刻见解，是如何有助于说明欧洲人与印第安人之间重大的、几乎是单向的病菌交流的。
NATURALLY, WE'RE DISPOSED to think about diseases just from our own point of view: what can we do to save ourselves and to kill the microbes Let's stamp out the scoundrels, and never mind what their motives are! In life in general, though, one has to understand the enemy in order to beat him, and that's especially true in medicine.
自然，我们都喜欢按照我们自己的观点来考虑疾病问题：我们怎样才能挽救自己和杀死病菌？让我们消灭这些坏蛋，而不必介意它们的动机是什么！然而，在一般的生活中，为了打败敌人，必须了解敌人，在医学中则尤其如此。
Hence let's begin by temporarily setting aside our human bias and considering disease from the microbes' point of view. After all, microbes are as much a product of natural selection as we are. What evolutionary benefit does a microbe derive from making us sick in bizarre ways, like giving us genital sores or diarrhea And why should microbes evolve so as to kill us That seems especially puzzling and self-defeating, since a microbe that kills its host kills itself.
因此，让我们首先暂时把我们人类的偏见放在一边，从病菌的角度来考虑疾病问题。要知道，病菌同我们一样都是自然选择的产物。病菌以各种稀奇古怪的方式使我们生病，如使我们得生殖器溃疡或腹泻。它这样做会得到什么样的演化利益呢？这似乎是特别令人费解而又自拆台脚的事，因为病菌杀死了它的宿主也就杀死了它自己。
Basically, microbes evolve like other species. Evolution selects for those individuals most effective at producing babies and at helping them spread to suitable places to live. For a microbe, spread may be defined mathematically as the number of new victims infected per each original patient. That number depends on how long each victim remains capable of infecting new victims, and how efficiently the microbe is transferred from one victim to the next.
从根本上说，病菌的演化和其他物种没有什么两样。演化所选择的，是那些在繁殖后代和帮助后代向适于生存的地方传播方面都是最有效的个体。可以从数学上把病菌的传播定义为：由每一个原发病人传染的新的受害者的数目。这个数目的大小取决于每一个受害者能够传染给新的受害者的持续时间的长短，以及这种病菌从一个受害者转移到下一个受害者的效率的高低。
Microbes have evolved diverse ways of spreading from one person to another, and from animals to people. The germ that spreads better leaves more babies and ends up favored by natural selection. Many of our “symptoms” of disease actually represent ways in which some damned clever microbe modifies our bodies or our behavior such that we become enlisted to spread microbes.
病菌已演化出各种不同的方式，从一个人传播给另一个人，以及从动物传播给人。传播能力强的病菌繁殖的后代也多，结果就会得到自然选择的偏爱。我们的疾病的许多“症状”，实际上不过是某种非常聪明的病菌在改变我们的身体或行为以便使我们帮助传播病菌时所表现出来的一些方式而已。
The most effortless way a germ could spread is by just waiting to be transmitted passively to the next victim. That's the strategy practiced by microbes that wait for one host to be eaten by the next host: for instance, salmonella bacteria, which we contract by eating already infected eggs or meat; the worm responsible for trichinosis, which gets from pigs to us by waiting for us to kill the pig and eat it without proper cooking; and the worm causing anisakiasis, with which sushi-loving Japanese and Americans occasionally infect themselves by consuming raw fish. Those parasites pass to a person from an eaten animal, but the virus causing laughing sickness (kuru) in the New Guinea highlands used to pass to a person from another person who was eaten. It was transmitted by cannibalism, when highland babies made the fatal mistake of licking their fingers after playing with raw brains that their mothers had just cut out of dead kuru victims awaiting cooking.
病菌传播的最不费力的方式，就是等待着被动地传染给下一个受害者。有些病菌等待一个宿主被下一个宿主吃掉，就是这种策略的运用：例如，沙门氏菌就是因为我们吃了已被感染的蛋或肉而感染上的；引起旋毛虫病的寄生虫是等我们在杀猪后吃了未煮熟的猪肉而从猪身上进入我们体内的；引起线虫肉芽病的寄生虫是喜吃寿司的日本人和美国人因为吃了生鱼片而有时感染上的。这些寄生虫都是从被吃的动物传递给人的，但引起新几内亚高原地区的强笑病(库鲁病 [1])的病毒通常是从一个被吃的人传递给另一个人的。这种病是通过吃人肉传播的：高原地区的母亲们把死于库鲁病的人的脑髓挖出来等待下锅，一旁的孩子把这未煮过的脑髓用手摆弄后舔了舔手指，从而犯下了致命的错误。
Some microbes don't wait for the old host to die and get eaten, but instead hitchhike in the saliva of an insect that bites the old host and flies off to find a new host. The free ride may be provided by mosquitoes, fleas, lice, or tsetse flies that spread malaria, plague, typhus, or sleeping sickness, respectively. The dirtiest of all tricks for passive carriage is perpetrated by microbes that pass from a woman to her fetus and thereby infect babies already at birth. By playing that trick, the microbes responsible for syphilis, rubella, and now AIDS pose ethical dilemmas with which believers in a fundamentally just universe have had to struggle desperately.
有些病菌不是等到旧宿主死后被吃掉，而是在昆虫的唾液中“搭便车”，这个昆虫咬了原来的宿主，然后赶紧离开去寻找新的宿主。提供这种“免费搭车”的可能是蚊子、跳蚤、虱子或采采蝇，它们分别传播疟疾、瘟疫、伤寒或昏睡病。这种被动传播的最卑鄙的把戏就是病菌干的，它们通过妇女传给胎儿，从而使出生后的婴儿受到感染。引起梅毒、风疹和现在的艾滋病的病菌就是靠玩这种把戏造成了道德的困境，而一些主张应该有一个基本正确的世界的人，不得不同这种困境作殊死的斗争。
Other germs take matters into their own hands, figuratively speaking. They modify the anatomy or habits of their host in such a way as to accelerate their transmission. From our perspective, the open genital sores caused by venereal diseases like syphilis are a vile indignity. From the microbes' point of view, however, they're just a useful device to enlist a host's help in inoculating microbes into a body cavity of a new host. The skin lesions caused by smallpox similarly spread microbes by direct or indirect body contact (occasionally very indirect, as when U.S. whites bent on wiping out “belligerent” Native Americans sent them gifts of blankets previously used by smallpox patients).
其他一些病菌可以说是由自己来处理事情。它们改变宿主的结构和习惯，来加速自己的传播。从我们的角度看，得了由梅毒之类性病引起的外露的生殖器溃疡是一种极不光彩的事。然而，从病菌的观点看，它们只是一种有用的手段，用以获得宿主的帮助把病菌移入新宿主的体腔。天花造成的皮肤损伤，同样是通过直接或间接的身体接触来传播病菌的(有时是十分间接的，如一心想要消灭“好斗的”印第安人的美国白人把天花患者以前用过的毯子作为礼物送给他们)。
More vigorous yet is the strategy practiced by the influenza, common cold, and pertussis (whooping cough) microbes, which induce the victim to cough or sneeze, thereby launching a cloud of microbes toward prospective new hosts. Similarly, the cholera bacterium induces in its victim a massive diarrhea that delivers bacteria into the water supplies of potential new victims, while the virus responsible for Korean hemorrhagic fever broadcasts itself in the urine of mice. For modification of a host's behavior, nothing matches rabies virus, which not only gets into the saliva of an infected dog but drives the dog into a frenzy of biting and thus infecting many new victims. But for physical effort on the bug's own part, the prize still goes to worms such as hookworms and schistosomes, which actively burrow through a host's skin from the water or soil into which their larvae had been excreted in a previous victim's feces.
流行性感冒、普通感冒和百日咳病菌所运用的策略就更厉害了，它们诱使受害者咳嗽或打喷嚏，把一群病菌向未来的新宿主喷射出去。同样，霍乱菌促使它的受害者大量腹泻，把病菌送入潜在的新受害者饮用的水源。引起朝鲜出血热的病毒通过鼠尿来传播。在改变宿主的行为方面，再没有什么能和狂犬病病毒相比的了，这种病毒不但进入了受到感染的狗的唾液中，而且还驱使这只狗疯狂地乱咬，从而使许多新的受害者受到感染。但就这种小虫子所作的实际努力来说，应该得奖的还是钩虫和血吸虫之类的寄生虫。它们的幼虫通过前一个受害者的粪便被排泄到水里或土里，又从那里努力地钻进新宿主的皮肤。
Thus, from our point of view, genital sores, diarrhea, and coughing are “symptoms of disease.” From a germ's point of view, they're clever evolutionary strategies to broadcast the germ. That's why it's in the germ's interests to “make us sick.” But why should a germ evolve the apparently self-defeating strategy of killing its host
因此，从我们的观点来看，生殖器溃疡、腹泻和咳嗽都是“症状”。但从病菌的观点看，它们就是传播病菌的聪明的演化策略。这就是为什么“使我们生病”是符合病菌的利益的。但是，为什么病菌会演化出杀死宿主这种明显自拆台脚的策略呢？
From the germ's perspective, that's just an unintended by-product (fat consolation to us!) of host symptoms promoting efficient transmission of microbes. Yes, an untreated cholera patient may eventually die from producing diarrheal fluid at a rate of several gallons per day. At least for a while, though, as long as the patient is still alive, the cholera bacterium profits from being massively broadcast into the water supplies of its next victims. Provided that each victim thereby infects on the average more than one new victim, the bacterium will spread, even though the first host happens to die.
从病菌的角度看，那只是宿主症状促进病菌高效传播的一个无心的附带结果(对我们来说真是一个莫大的安慰！)。是的，一个没有得到治疗的霍乱病人，最后可能因为每天拉稀达几加仑而送命。然而，至少在一段时间里，只要这病人仍然活着，霍乱菌就会由于大量传播进下一个受害者的饮用水源而得到好处。倘若每个受害者因而平均感染一个以上的新的受害者，那么即使第一个宿主碰巧死了，霍乱菌仍然会传播开去。
SO MUCH FOR our dispassionate examination of the germ's interests. Now let's get back to considering our own selfish interests: to stay alive and healthy, best done by killing the damned germs. One common response of ours to infection is to develop a fever. Again, we're used to considering fever as a “symptom of disease,” as if it developed inevitably without serving any function. But regulation of body temperature is under our genetic control and doesn't just happen by accident. A few microbes are more sensitive to heat than our own bodies are. By raising our body temperature, we in effect try to bake the germs to death before we get baked ourselves.
我们对病菌利益的不带感情的考察就到此为止。现在，让我们回过头来考虑一下我们本身的自私的利益：活下去并保持健康，最好的办法就是杀死那些该死的病菌。我们受到感染的一个普遍反应是发烧。而我们又一次在习惯上把发烧看作是一种“症状”，好像就这样无缘无故地照例发生了。但是，体温的调节是受到基因控制的，并不是无缘无故发生的。有些病对热的反应比我们的身体更敏感。提高我们的体温，实际上就是要在烤死我们自己之前把病菌烤死。
Another common response of ours is to mobilize our immune system. White blood cells and other cells of ours actively seek out and kill foreign microbes. The specific antibodies that we gradually build up against a particular microbe infecting us make us less likely to get reinfected once we become cured. As we all know from experience, there are some illnesses, such as flu and the common cold, to which our resistance is only temporary; we can eventually contract the illness again. Against other illnesses, though—including measles, mumps, rubella, pertussis, and the now defeated smallpox—our antibodies stimulated by one infection confer lifelong immunity. That's the principle of vaccination: to stimulate our antibody production without our having to go through the actual experience of the disease, by inoculating us with a dead or weakened strain of microbe.
我们的另一个普遍反应就是把我们的免疫系统动员起来。我们的白血细胞和其他细胞积极地搜出并杀死外来病菌。我们在抵抗某种使我们受到感染的病菌的过程中逐步形成的特定的抗体，使我们在痊愈后不大可能再次受到感染。我们根据经验都知道，有些病如流行性感冒和普通感冒，我们对它们的抵抗力只是暂时的；我们最后还是有可能再次感染上这种病的。然而，对其他一些疾病——包括麻疹、流行性腮腺炎、风疹、百日咳以及现在已被战胜的天花——我们的由一次感染激发起来的抗体使我们获得终生免疫。这就是预防接种的原理：给我们接种一种已死的或变弱了的菌株，促使我们的抗体产生，而不必真的去生病。
Alas, some clever microbes don't just cave in to our immune defenses. Some have learned to trick us by changing those molecular pieces of the microbe (its so-called antigens) that our antibodies recognize. The constant evolution or recycling of new strains of flu, with differing antigens, explains why your having gotten flu two years ago didn't protect you against the different strain that arrived this year. Malaria and sleeping sickness are even more slippery customers in their ability rapidly to change their antigens. Among the slipperiest of all is AIDS, which evolves new antigens even as it sits within an individual patient, thereby eventually overwhelming his or her immune system.
可是，有些聪明的病菌在我们的免疫防御面前就是不屈服。有些已学会了改变我们的抗体能认出来的那一细菌的某些分子结构(即所谓的抗原)来使我们上当。新品种的流行性感冒通过不断的演化或改造，产生了不同的抗原，这就是为什么虽然你在两年前得过流感，但在今年另一种流感到来时你仍不能免于感染的缘故。疟疾和昏睡病由于有迅速改变抗原的能耐，成了甚至更难抓住的主顾。最难抓住的是艾滋病，因为它甚至在一个病人的体内也能演化出新的抗原，从而破坏了这个病人的免疫系统。
Our slowest defensive response is through natural selection, which changes our gene frequencies from generation to generation. For almost any disease, some people prove to be genetically more resistant than are others. In an epidemic those people with genes for resistance to that particular microbe are more likely to survive than are people lacking such genes. As a result, over the course of history, human populations repeatedly exposed to a particular pathogen have come to consist of a higher proportion of individuals with those genes for resistance—just because unfortunate individuals without the genes were less likely to survive to pass their genes on to babies.
我们最缓慢的防御反应是通过自然选择表现出来的。自然选择改变了我们一代代的基因频率。对于几乎任何一种疾病来说，某些人证明比另一些人具有更强的基因抵抗能力。在疾病流行时，那些具有抵抗某种病菌的基因的人，比缺乏这种基因的人更有可能生存下来。因此，在历史的进程中，在反复接触某种病原体的人口中，具有那些抗病基因的个体的人数比例较高——这完全是因为没有这种基因的不幸的个体不大可能生存下来把他们的基因传给后代。
Fat consolation, you may be thinking again. This evolutionary response is not one that does the genetically susceptible dying individual any good. It does mean, though, that a human population as a whole becomes better protected against the pathogen. Examples of those genetic defenses include the protections (at a price) that the sickle-cell gene, Tay-Sachs gene, and cystic fibrosis gene may confer on African blacks, Ashkenazi Jews, and northern Europeans against malaria, tuberculosis, and bacterial diarrheas, respectively.
你可能又一次认为，这是莫大的安慰。这种演化反应对基因易受感染的行将消失的个体没有任何好处。然而，这的确意味着整个人口有了抵抗这种病原体的更强的能力。关于这种基因防御的例子如：镰状红细胞基因、泰萨二氏病基因和囊性纤维变性基因可能使非洲黑人、德系犹太人和北欧人分别获得了保护自己(以一定的代价)抵抗疟疾、肺结核和细菌性腹泻的能力。
In short, our interaction with most species, as exemplified by hummingbirds, doesn't make us or the hummingbird “sick.” Neither we nor hummingbirds have had to evolve defenses against each other. That peaceful relationship was able to persist because hummingbirds don't count on us to spread their babies or to offer our bodies for food. Hummingbirds evolved instead to feed on nectar and insects, which they find by using their own wings.
总之，我们同大多数物种的相互作用，就像我们同蜂鸟的关系所证明的那样，不会使我们“生病”，也不会使蜂鸟“生病”。无论是我们还是蜂鸟，都不需演化出相互防范的能力。这种和平的关系能够维持下去，因为蜂鸟不指望我们为它们传播后代，也不指望我们把身体给它们当食物。蜂鸟演化的结果是它们以花蜜和昆虫为食，而这些东西是它们靠运用翅膀得来的。
But microbes evolved to feed on the nutrients within our own bodies, and they don't have wings to let them reach a new victim's body once the original victim is dead or resistant. Hence many germs have had to evolve tricks to let them spread between potential victims, and many of those tricks are what we experience as “symptoms of disease.” We've evolved countertricks of our own, to which the germs have responded by evolving counter-countertricks. We and our pathogens are now locked in an escalating evolutionary contest, with the death of one contestant the price of defeat, and with natural selection playing the role of umpire. Now let's consider the form of the contest: blitzkrieg or guerrilla war
但是，病菌演化的结果却是以我们体内的养料为食，一旦原来的受害者死了或者产生了抵抗力，它们也没有翅膀可以让它们飞到一个新的受害者的身体上去。因此，许多病菌不得不演化出一些花招，好让它们在潜在的受害者之间进行传播，而许多这样的花招也就是我们身上所体现出来的“症状”。我们也已演化出我们自己的反花招，对此细菌又演化出反反花招来予以回答。我们和我们的病原体现在在一场逐步升级的演化竞赛中难解难分，以竞赛一方的死亡为失败的代价，而自然选择就是这场竞赛的裁判。现在就让我考虑一下这场竞赛的形式：是闪电战还是游击战？
SUPPOSE THAT ONE counts the number of cases of some particular infectious disease in some geographic area, and watches how the numbers change with time. The resulting patterns differ greatly among diseases. For certain diseases, like malaria or hookworm, new cases appear any month of any year in an affected area. So-called epidemic diseases, though, produce no cases for a long time, then a whole wave of cases, then no more cases again for a while.
假定我们计算一下某个地区某种传染病病例的数目，并注意这些数目如何随时间而变化。由此而产生的变化模式在各类疾病中是大不相同的。对某些疾病如疟疾或钩虫病来说，在一个受侵袭的地区，任何一年的任何一个月都会有新的病例出现。然而，所谓流行疾病在一个很长时间里可能连一例都没有，然后是一大批病例，接着有一阵子又没有任何病例。
Among such epidemic diseases, influenza is one personally familiar to most Americans, certain years being particularly bad years for us (but great years for the influenza virus). Cholera epidemics come at longer intervals, the 1991 Peruvian epidemic being the first one to reach the New World during the 20th century. Although today's influenza and cholera epidemics make front-page stories, epidemics used to be far more terrifying before the rise of modern medicine. The greatest single epidemic in human history was the one of influenza that killed 21 million people at the end of the First World War. The Black Death (bubonic plague) killed one-quarter of Europe's population between 1346 and 1352, with death tolls ranging up to 70 percent in some cities. When the Canadian Pacific Railroad was being built through Saskatchewan in the early 1880s, that province's Native Americans, who had previously had little exposure to whites and their germs, died of tuberculosis at the incredible rate of 9 percent per year.
在这些流行疾病中，流行性感冒是大多数美国人因有亲身经历而非常熟悉的一种病，有几年对我们来说特别糟糕(但对流行性感冒病毒来说则是美好的年头)。霍乱这种流行病发生的间隔时间较长，1991年秘鲁的霍乱是20世纪首次到达新大陆的流行病。虽然今天流行性感冒和霍乱的流行成了报纸的头版新闻报道，但在现代医药出现前的流行病通常要可怕得多。人类历史上最大的一次流行病是在第一次世界大战结束时杀死2100万人的流行性感冒。黑死病(腺鼠疫)在1346年到1352年间杀死了欧洲四分之一的人口，在有些城市里死亡人数高达70%。19世纪80年代初，当加拿大太平洋铁路修经萨斯喀彻温时，该省以前很少接触过白人及其病菌的印第安人死于肺结核的人数每年竟达到惊人的9%。
The infectious diseases that visit us as epidemics, rather than as a steady trickle of cases, share several characteristics. First, they spread quickly and efficiently from an infected person to nearby healthy people, with the result that the whole population gets exposed within a short time. Second, they're “acute” illnesses: within a short time, you either die or recover completely. Third, the fortunate ones of us who do recover develop antibodies that leave us immune against a recurrence of the disease for a long time, possibly for the rest of our life. Finally, these diseases tend to be restricted to humans; the microbes causing them tend not to live in the soil or in other animals. All four of these traits apply to what Americans think of as the familiar acute epidemic diseases of childhood, including measles, rubella, mumps, pertussis, and smallpox.
作为流行病而不是作为点滴的小病光顾我们的这些传染病有几个共同的特点。首先，它们从一个受感染的人迅速而高效地传给近旁健康的人，结果使整个人口在很短时间内受到感染。其次，它们都是“急性”病：在很短时间内，你要么死掉，要么完全康复。第三，我们当中的确获得康复的那些幸运的人产生了抗体，使我们在很长时间内，也可能是一辈子不用担心这种病会复发。最后，这些病往往只在人类中传播；引起这些病的病菌往往不是生活在土壤中或其他动物身上。所有这4个特点也适用于美国人所认为的那些习见的儿童急性传染病，其中包括麻疹、风疹、急性腮腺炎、百日咳和天花。
The reason why the combination of those four traits tends to make a disease run in epidemics is easy to understand. In simplified form, here's what happens. The rapid spread of microbes, and the rapid course of symptoms, mean that everybody in a local human population is quickly infected and soon thereafter is either dead or else recovered and immune. No one is left alive who could still be infected. But since the microbe can't survive except in the bodies of living people, the disease dies out, until a new crop of babies reaches the susceptible age—and until an infectious person arrives from the outside to start a new epidemic.
这4个特点结合起来往往造成了某种疾病的流行，其原因不难理解。简单地说，情况是这样的：病菌的迅速传播和症状的迅速发展，意味着当地人口中的每一个人很快就受到感染，之后不久他或者死去，或者康复并获得免疫力。仍然会受到感染的人都不会活下来。但由于这种病菌除了在活人体内是不可能生存的，所以人死了这种病也就消失了，直到又一批后代达到易受感染的年代——直到一个受到感染的外来人使一场流行病重新开始。
A classic illustration of how such diseases occur as epidemics is the history of measles on the isolated Atlantic islands called the Faeroes. A severe epidemic of measles reached the Faeroes in 1781 and then died out, leaving the islands measles free until an infected carpenter arrived on a ship from Denmark in 1846. Within three months, almost the whole Faeroes population (7,782 people) had gotten measles and then either died or recovered, leaving the measles virus to disappear once again until the next epidemic. Studies show that measles is likely to die out in any human population numbering fewer than half a million people. Only in larger populations can the disease shift from one local area to another, thereby persisting until enough babies have been born in the originally infected area that measles can return there.
关于这些疾病是怎样流行起来的，有一个典型的事例是大西洋上叫做法罗群岛 [2]的与世隔绝的岛屿上的麻疹病史。1781年，一次严重的麻疹流行病到达法罗群岛，接着又消失了，其后该群岛就不再有麻疹发生，直到1846年，一个受到感染的木匠从丹麦坐船到来。不出3个月，法罗群岛的几乎全部人口(7782人)都得了麻疹，于是有的人死去，有的人康复，麻疹病毒又一次消失，直到下一次流行。一些研究表明，麻疹可能会在任何少于50万人的人口中消失。只有在比较多的人口中，这种病才会从一个地区转移到另一个地区，直到原先受感染地区里出生的婴儿达到足够的数目，麻疹又会卷土重来。
What's true for measles in the Faeroes is true of our other familiar acute infectious diseases throughout the world. To sustain themselves, they need a human population that is sufficiently numerous, and sufficiently densely packed, that a numerous new crop of susceptible children is available for infection by the time the disease would otherwise be waning. Hence measles and similar diseases are also known as crowd diseases.
适用于法罗群岛上麻疹的情况，也适用于世界上其他一些我们所熟悉的急性传染病。为了维持自身的存在，这些病需要有足够多的人口，足够拥挤的稠密人口，这样，到这种病不然就会衰退的时候，又有一批众多的易受感染的儿童现成可供感染。因此，麻疹和一些类似的疾病也叫做群众疾病。
OBVIOUSLY, CROWD DISEASES could not sustain themselves in small bands of hunter-gatherers and slash-and-burn farmers. As tragic modern experience with Amazonian Indians and Pacific Islanders confirms, almost an entire tribelet may be wiped out by an epidemic brought by an outside visitor—because no one in the tribelet had any antibodies against the microbe. For example, in the winter of 1902 a dysentery epidemic brought by a sailor on the whaling ship Active killed 51 out of the 56 Sadlermiut Eskimos, a very isolated band of people living on Southampton Island in the Canadian Arctic. In addition, measles and some of our other “childhood” diseases are more likely to kill infected adults than children, and all adults in the tribelet are susceptible. (In contrast, modern Americans rarely contract measles as adults, because most of them get either measles or the vaccine against it as children.) Having killed most of the tribelet, the epidemic then disappears. The small population size of tribelets explains not only why they can't sustain epidemics introduced from the outside, but also why they never could evolve epidemic diseases of their own to give back to visitors.
显然，群众疾病不可能在小群狩猎采集族群和刀耕火种的农民中存在。现代亚马孙河地区印第安人和太平洋岛民的悲惨经历表明，一个小部落可能被一个外来人带来的一种流行病几乎全部消灭——因为这个小部落中没有一个人有任何抵抗这种病菌的抗体。例如，1902年冬天，由捕鲸船“活跃”号上的一个水手带来的一场痢疾流行使56个萨德勒缪特爱斯基摩人中的51个人丧生，这是生活在加拿大北极地区南安普顿岛上的一群完全与世隔绝的人。此外，麻疹和我们的其他一些“童年”病，杀死受感染的成年人比杀死儿童的可能性更大，而那个小部落里的成年人又全都是易受感染的。(相比之下，现代的美国成年人很少有感染上麻疹的，因为他们中大多数在童年时或者得过麻疹，或者接受过预防接种。)那场流行病在把那个小部落中的大多数人杀死后接着就消失了。小部落人口少，这一点不但说明了为什么他们承受不住从外面带来的流行病，而且也说明了为什么他们没有能演化出自己的流行病去回敬外来人。
That's not to say, though, that small human populations are free from all infectious diseases. They do have infections, but only of certain types. Some are caused by microbes capable of maintaining themselves in animals or in the soil, with the result that the disease doesn't die out but remains constantly available to infect people. For example, the yellow fever virus is carried by African wild monkeys, whence it can always infect rural human populations of Africa, whence it was carried by the transatlantic slave trade to infect New World monkeys and people.
然而，这并不是说人口少就百病不生。他们同样会得传染病，不过只限于几种传染病而已。有些传染病是由能在动物身上或土壤中生存的病菌引起的，结果这种病不会消失，而且始终可以使人受到感染。例如，黄热病病毒是由非洲野猴携带的，它总是能够通过野猴感染非洲的农村人口，再从这些人通过横渡大西洋的奴隶贸易带去感染新大陆的猴子和人。
Still other infections of small human populations are chronic diseases such as leprosy and yaws. Since the disease may take a very long time to kill its victim, the victim remains alive as a reservoir of microbes to infect other members of the tribelet. For instance, the Karimui Basim of the New Guinea highlands, where I worked in the 1960s, was occupied by an isolated population of a few thousand people, suffering from the world's highest incidence of leprosy—about 40 percent! Finally, small human populations are also susceptible to nonfatal infections against which we don't develop immunity, with the result that the same person can become reinfected after recovering. That happens with hookworm and many other parasites.
还有一些传染病发生在人口稀少的地方，它们是麻风和雅司病 [3]之类的慢性病。由于这种病可能要花很长时间才能杀死它的患者，所以患者在活着时就成了感染这个小部落的其他成员的病菌仓库。例如，我在60年代曾在新几内亚高原地区的卡里穆伊巴西姆工作过，那里的居民是几千个与世隔绝的人，他们的麻风病发生率是全世界最高的——约40%！人口少的群体毕竟也是容易得一些非致命的传染病的。由于我们对这种传染病没有形成免疫力，结果同一个人在康复之后仍会再度感染。钩虫和其他许多寄生虫的情况就是如此。
All these types of diseases, characteristic of small isolated populations, must be the oldest diseases of humanity. They were the ones we could evolve and sustain through the early millions of years of our evolutionary history, when the total human population was tiny and fragmented. These diseases are also shared with, or similar to the diseases of, our closest wild relatives, the African great apes. In contrast, the crowd diseases, which we discussed earlier, could have arisen only with the buildup of large, dense human populations. That buildup began with the rise of agriculture starting about 10,000 years ago and then accelerated with the rise of cities starting several thousand years ago. In fact, the first attested dates for many familiar infectious diseases are surprisingly recent: around 1600 B.C. for smallpox (as deduced from pockmarks on an Egyptian mummy), 400 B.C. for mumps, 200 B.C. for leprosy, A.D. 1840 for epidemic polio, and 1959 for AIDS.
所有这些为与世隔绝的很少人口所特有的疾病，谅必都是人类最古老的疾病。它们是我们在早期几百万年的进化史中得以形成并保持的疾病，因为那时的总人口为数甚少而且零星分散。这些疾病是我们与我们的野生近亲非洲类人猿所共有的，或者与它们的疾病相类似的。相比之下，我们前面所讨论的那种群众疾病只有在积聚起众多的稠密人口时才可能出现。这种人口的积聚，随着大约1万年前农业的开始出现而出现，然后又随着几千年前城市的开始出现而加速发展。事实上，许多为人们所熟悉的传染病的得到证实的最早出现年代，竟晚得令人惊奇：天花出现在公元前1600年左右(从一具埃及木乃伊身上的痘痕推断出来)，流行性腮腺炎出现在公元前400年，麻风出现在公元前200年，流行性脊髓灰质炎出现在公元1840年，艾滋病出现在1959年。
WHY DID THE rise of agriculture launch the evolution of our crowd infectious diseases One reason just mentioned is that agriculture sustains much higher human population densities than does the hunting-gathering lifestyle—on the average, 10 to 100 times higher. In addition, hunter-gatherers frequently shift camp and leave behind their own piles of feces with accumulated microbes and worm larvae. But farmers are sedentary and live amid their own sewage, thus providing microbes with a short path from one person's body into another's drinking water.
为什么农业的出现会成为我们群众传染病形成的开端？其中一个原因前面已经提到，那就是农业比狩猎采集的生活方式维持了高得多的人口密度——平均要高10倍到100倍。另外，狩猎采集族群经常变换营地，留下了一堆堆排泄物，上面聚集了大量病菌和寄生虫的幼虫。但农民是定居的，他们生活在自己排放出来的污水之中，从而为病菌从一个人的身体进入另一个人的饮用水源提供了捷径。
Some farming populations make it even easier for their own fecal bacteria and worms to infect new victims, by gathering their feces and urine and spreading them as fertilizer on the fields where people work. Irrigation agriculture and fish farming provide ideal living conditions for the snails carrying schistosomiasis and for flukes that burrow through our skin as we wade through the feces-laden water. Sedentary farmers become surrounded not only by their feces but also by disease transmitting rodents, attracted by the farmers' stored food. The forest clearings made by African farmers also provide ideal breeding habitats for malaria-transmitting mosquitoes.
有些农业人口把自己的粪便收集起来，当作肥料撒到人们劳动的田里，从而使粪便中的病菌和寄生虫去感染新的受害者变得甚至更加容易。灌溉农业和鱼类养殖为蜗牛和水蛭提供了理想的生活环境。蜗牛是吸血虫的宿主，而水蛭则在我们涉过满是粪便的水中时钻进我们的皮肤。定居的农民周围不但有自己的粪便，而且还有被他们贮藏的粮食吸引来的传播疾病的啮齿目动物。非洲农民砍伐出来的林中空地也为疟蚊提供了理想的滋生地。
If the rise of farming was thus a bonanza for our microbes, the rise of cities was a greater one, as still more densely packed human populations festered under even worse sanitation conditions. Not until the beginning of the 20th century did Europe's urban populations finally become self-sustaining: before then, constant immigration of healthy peasants from the countryside was necessary to make up for the constant deaths of city dwellers from crowd diseases. Another bonanza was the development of world trade routes, which by Roman times effectively joined the populations of Europe, Asia, and North Africa into one giant breeding ground for microbes. That's when smallpox finally reached Rome, as the Plague of Antoninus, which killed millions of Roman citizens between A.D. 165 and 180.
如果说农业的出现就这样地使我们的病菌交了好运，那么城市的出现则给它们带来了更大的幸运，因为在甚至更糟的卫生条件下，更加拥挤的稠密人口使情况恶化了。直到20世纪初，欧洲的城市人口才最后稳定下来：在那以前，来自农村的健康农民不断地移居城市，这对于补充城市中因群众疾病而死去的人是必要的。对病菌来说，另一件好事是世界贸易路线的发展，到罗马时代，这些贸易路线把欧洲、亚洲和北非有效地连接成一个巨大的病菌繁殖场。也就是在这个时候，所谓安东尼瘟疫的天花终于到达罗马，在公元165年到180年期间杀死了几百万罗马城镇居民。
Similarly, bubonic plague first appeared in Europe as the Plague of Justinian (A.D. 542–43). But plague didn't begin to hit Europe with full force as the Black Death epidemics until A.D. 1346, when a new route for overland trade with China provided rapid transit, along Eurasia's east-west axis, for flea-infested furs from plague-ridden areas of Central Asia to Europe. Today, our jet planes have made even the longest intercontinental flights briefer than the duration of any human infectious disease. That's how an Aerolineas Argentinas airplane, stopping in Lima (Peru) in 1991, managed to deliver dozens of cholera-infected people that same day to my city of Los Angeles, over 3,000 miles from Lima. The explosive increase in world travel by Americans, and in immigration to the United States, is turning us into another melting pot—this time, of microbes that we previously dismissed as just causing exotic diseases in far-off countries.
同样，所谓查士丁尼瘟疫的腺鼠疫也第一次在欧洲出现了(公元542—543年)。但直到公元1346年，所谓黑死病的鼠疫才开始全力打击欧洲，那时一条新的与中国的陆上贸易路线，为满是跳蚤的毛皮提供了一条沿欧亚大陆东西轴线，从到处瘟疫的中亚地区到欧洲的快速运输通道。今天，我们的喷气飞机使得甚至最长的洲际飞行比人类任何传染病的持续时间都要短暂。这就是1991年一架停在利马(秘鲁)的阿根廷航空公司的飞机如何设法做到从利马飞越3000英里把几十个感染霍乱的人当天送到我所居住的城市洛杉矶。美国人周游世界和外国人移居美国的迅速增多，正在把我们变成另一座熔炉——这一次是病菌的熔炉，而这些病菌我们原先认为不过是在遥远的国度引起一些古怪的疾病而未曾予以理会。
THUS, WHEN THE human population became sufficiently large and concentrated, we reached the stage in our history at which we could at last evolve and sustain crowd diseases confined to our own species. But that conclusion presents a paradox: such diseases could never have existed before then! Instead, they had to evolve as new diseases. Where did those new diseases come from
因此，当人口的数量和集中达到一定程度时，我们也就达到了这样的一个历史阶段，在这个阶段我们至少能够形成并保持只有我们人类才会有的群众疾病。但这个结论也有其矛盾之处：在那时以前这些病是不可能存在的！相反，它们必须演化成新的疾病。那么，这些新的疾病又是从哪里来的呢？
Evidence has recently been emerging from molecular studies of the disease-causing microbes themselves. For many of the microbes responsible for our unique diseases, molecular biologists can now identify the microbe's closest relatives. These also prove to be agents of crowd infectious diseases—but ones confined to various species of our domestic animals and pets! Among animals, too, epidemic diseases require large, dense populations and don't afflict just any animal: they're confined mainly to social animals providing the necessary large populations. Hence when we domesticated social animals, such as cows and pigs, they were already afflicted by epidemic diseases just waiting to be transferred to us.
最近，由于对致病病菌所进行的分子研究，证据正在不断出现。就引起我们独有的疾病的许多病菌来说，分子生物学家现在能够确定一些亲缘关系最为接近的病菌。这些病菌同样证明是群众传染病的媒介——不过只在我们的各种家畜和宠物中流行罢了！在动物中，流行病同样需要稠密的大种群，而不是只去折磨任何某一只动物：这些流行病主要地只发生在需要有大的种群的群居动物中。因此，当我们驯养牛和猪这类群居动物时，它们已经受到了一些流行病的折磨，只不过在等待着转移给我们罢了。
For example, measles virus is most closely related to the virus causing rinderpest. That nasty epidemic disease affects cattle and many wild cud-chewing mammals, but not humans. Measles in turn doesn't afflict cattle. The close similarity of the measles virus to the rinderpest virus suggests that the latter transferred from cattle to humans and then evolved into the measles virus by changing its properties to adapt to us. That transfer is not at all surprising, considering that many peasant farmers live and sleep close to cows and their feces, urine, breath, sores, and blood. Our intimacy with cattle has been going on for the 9,000 years since we domesticated them—ample time for the rinderpest virus to discover us nearby. As Table 11.1 illustrates, others of our familiar infectious diseases can similarly be traced back to diseases of our animal friends.
例如，麻疹病毒同牛瘟病毒亲缘关系最为接近。这种可怕的流行病侵袭牛和许多野生的反刍哺乳动物，但不侵袭人。反过来，麻疹也不侵袭牛。麻疹病毒和牛瘟病毒极其相似这一点表明，后者从牛转移给人，然后通过改变其特性以适应人的情况而演化成麻疹病毒。考虑到许多农民的生活和睡眠同牛及其粪便、呼吸、溃疡和血液近在咫尺，这种转移就一点也不令人奇怪了。自从我们对牛驯养以来，我们和牛的这种亲密关系已存在了9000年之久——这大量时间足以使牛瘟病毒发现我们就在它的近旁。如表11.1所示，其他一些我们所熟悉的传染病同样可以追溯到我们的动物朋友身上的疾病。
| TABLE II. I | Deadly Gifts from Our Animal Friends |
| Human Disease | Animal with Most Closely Related Pathogen |
| 人类疾病 | 携带关系最为接近的抗原体的动物 |
| Measles | cattle (rinderpest) |
| 麻疹 | 牛(牛瘟) |
| Tuberculosis | cattle |
| 肺结核 | 牛 |
| Smallpox | cattle (cowpox) or other livestock with related pox viruses |
| 天花 | 牛(牛痘)或携带亲缘痘病毒的其他牲畜 |
| Flu | pigs and ducks |
| 流行性感冒 | 猪和鸭 |
| Pertussis | pigs, dogs |
| 百日咳 | 猪和狗 |
| Falciparum malaria | birds (chickens and ducks ) |
| 恶性疟疾 | 禽鸟(鸡和鸭？) |
TABLE 11.1 Deadly Gifts from Our Animal Friends
表11.1 来自我们的动物朋友的致命礼物
GIVEN OUR PROXIMITY to the animals we love, we must be getting constantly bombarded by their microbes. Those invaders get winnowed by natural selection, and only a few of them succeed in establishing themselves as human diseases. A quick survey of current diseases lets us trace out four stages in the evolution of a specialized human disease from an animal precursor.
考虑到我们同我们所喜爱的动物的亲密关系，我们必定不断地受到它们的病菌的攻击。这些入侵者经过自然选择的筛选，只有少数得以成为人类的疾病。只要把当前的一些疾病迅速地观察一下，我们就可以看出原为动物疾病向人类转化疾病演化的4个阶段。
The first stage is illustrated by dozens of diseases that we now and then pick up directly from our pets and domestic animals. They include catscratch fever from our cats, leptospirosis from our dogs, psittacosis from our chickens and parrots, and brucellosis from our cattle. We're similarly liable to pick up diseases from wild animals, such as the tularemia that hunters can get from skinning wild rabbits. All those microbes are still at an early stage in their evolution into specialized human pathogens. They still don't get transmitted directly from one person to another, and even their transfer to us from animals remains uncommon.
第一阶段可以由几十种病作为例证，这些病是我们有时从我们的宠物和家畜那里直接得来的。它们包括从我们的猫那里得来的猫抓热，从我们的狗那里得来的钩端螺旋体病，从我们的鸡和鹦鹉那里得来的鹦鹉热，以及从我们的牛那里得来的布鲁氏菌病。我们同样也会从野生动物那里感染疾病，例如猎人在剥野兔皮时可能得兔热病。所有这些病菌仍然处在向人类转化病原体演化的早期阶段。它们仍然不能直接地从一个人传染给另一个人，甚至它们从动物身上转移给我们也仍属罕见。
In the second stage a former animal pathogen evolves to the point where it does get transmitted directly between people and causes epidemics. However, the epidemic dies out for any of several reasons, such as being cured by modern medicine, or being stopped when everybody around has already been infected and either becomes immune or dies. For example, a previously unknown fever termed O'nyong-nyong fever appeared in East Africa in 1959 and proceeded to infect several million Africans. It probably arose from a virus of monkeys and was transmitted to humans by mosquitoes. The fact that patients recovered quickly and became immune to further attack helped the new disease die out quickly. Closer to home for Americans, Fort Bragg fever was the name applied to a new leptospiral disease that broke out in the United States in the summer of 1942 and soon disappeared.
在第二阶段，原先动物的病原体的演化已达到可以直接在人群中传播从而引起流行病的地步。然而，这种流行病由于几个原因而消失了，如被现代医药治愈了，或因周围的每一个人都已得过病了，有的获得了免疫力，有的已经死了。例如，以前有一种叫做奥尼翁-尼翁热的不明热病于1959年在非洲出现，接着感染了几百万非洲人。它大概是猴子身上的一种病毒引起的，由蚊子传染给人。病人很快康复并不会复发，这一点有助于这种新出现的病很快消失。美国人家乡的一种病叫做布雷格堡热，这是给一种新出现的钩端螺旋体病取的名字，这种病于1942年夏季在美国爆发，随后很快消失不见了。
A fatal disease vanishing for another reason was New Guinea's laughing sickness, transmitted by cannibalism and caused by a slow-acting virus from which no one has ever recovered. Kuru was on its way to exterminating New Guinea's Foré tribe of 20,000 people, until the establishment of Australian government control around 1959 ended cannibalism and thereby the transmission of kuru. The annals of medicine are full of accounts of diseases that sound like no disease known today, but that once caused terrifying epidemics and then disappeared as mysteriously as they had come. The “English sweating sickness,” which swept and terrified Europe between 1485 and 1552, and the “Picardy sweats” of 18th- and 19th-century France, are just two of the many epidemic illnesses that vanished long before modern medicine had devised methods for identifying the responsible microbes.
由于另一原因而消失不见的一种致命疾病是新几内亚的库鲁病。这种病因吃人肉而传染，是由一种作用缓慢的病毒引起的，人一旦染上这种病毒，就终生不会痊愈。就在库鲁病快要消灭新几内亚的2万人的福雷部落时，澳大利亚政府于1959年左右建立了对这一地区的管理，结束了吃人肉的习俗，从而也结束了库鲁病的传播。医学史连篇累牍地记载了一些我们今天闻所未闻的疾病，但这些病曾一度引起了令人恐怖的流行病，接着又像出现时那样神秘地消失得无影无踪。有许多流行病在现代医学发明出用以确定罪魁祸首的病菌的方法之前很久便已销声匿迹了，从1485年到1552年在欧洲迅速蔓延并使欧洲一片惊慌的“英国汗热病”和18、19世纪法国的“皮卡迪汗热病”只是其中两例而已。
A third stage in the evolution of our major diseases is represented by former animal pathogens that did establish themselves in humans, that have not (not yet ) died out, and that may or may not still become major killers of humanity. The future remains very uncertain for Lassa fever, caused by a virus derived probably from rodents. Lassa fever was first observed in 1969 in Nigeria, where it causes a fatal illness so contagious that Nigerian hospitals have been closed down if even a single case appears. Better established is Lyme disease, caused by a spirochete that we get from the bite of ticks carried by mice and deer. Although the first known human cases in the United States appeared only as recently as 1962, Lyme disease is already reaching epidemic proportions in many parts of our country. The future of AIDS, derived from monkey viruses and first documented in humans around 1959, is even more secure (from the virus's perspective).
我们主要疾病的演化的第三阶段可以原先的动物病原体为代表，这些病原体确已在人体内安家落户，但并没有(尚未？)消失，可能仍然是或可能仍然不是人类的主要杀手。拉沙热是由一种可能来自啮齿目动物的病毒引起的，它的前途仍然十分难以预料。拉沙热是1969年在尼日利亚观察到的，它在那里引起了一种传染性很强的致命疾病，即使出现一例这样的病，尼日利亚的医院就都得关闭。情况比较清楚的是莱姆病，它是由一种螺旋体引起的，老鼠和鹿携带的扁虱叮咬了人，这种螺旋体就从叮咬处进入人体。虽然人类感染莱姆病的已知首批病例晚至1962年才在美国出现，但在我国的许多地方，莱姆病已经达到了流行的程度。艾滋病来自猴子的病毒，1959年左右有了关于人类感染这种病的最早记录。这种病的前途甚至更有保障(从艾滋病毒的观点看)。
The final stage of this evolution is represented by the major, long-established epidemic diseases confined to humans. These diseases must have been the evolutionary survivors of far more pathogens that tried to make the jump to us from animals—and mostly failed.
这种演化的最后阶段可以只有人类才会感染的那些主要的由来已久的疾病为代表。这些疾病必定是多得多的病原体在演化过程中的幸存者，所有那些病原体都曾力图迅速转移到我们身上——但多半失败了。
What is actually going on in those stages, as an exclusive disease of animals transforms itself into an exclusive disease of humans One transformation involves a change of intermediate vector: when a microbe relying on some arthropod vector for transmission switches to a new host, the microbe may be forced to find a new arthropod as well. For example, typhus was initially transmitted between rats by rat fleas, which sufficed for a while to transfer typhus from rats to humans. Eventually, typhus microbes discovered that human body lice offered a much more efficient method of traveling directly between humans. Now that Americans have mostly deloused themselves, typhus has discovered a new route into us: by infecting eastern North American flying squirrels and then transferring to people whose attics harbor flying squirrels.
在这些阶段究竟发生了什么，使一种本来为动物所独有的疾病转化为一种为人类所独有的疾病？有一种转化涉及居中传病媒介的改变：如果一种依赖某种节肢动物为传播媒介的病菌要转移到一个新宿主身上去，这种病菌可能不得不也去寻找一种新的节肢动物。例如，斑疹伤寒最初是由老鼠身上的跳蚤在老鼠之间传播的，这些跳蚤过不多久就能把斑疹伤寒从老鼠身上转移到人的身上。最后，斑疹伤寒菌发现，人身上的虱子提供了一种效率高得多的在人与人之间直接往来的方法。由于美国人大都消灭了身上的虱子，斑疹伤寒又发现了进入我们体内的一条新的路线：先是传染给北美东部的飞鼠，这些飞鼠藏在阁楼上，然后再通过这些飞鼠传染给住户。
In short, diseases represent evolution in progress, and microbes adapt by natural selection to new hosts and vectors. But compared with cows' bodies, ours offer different immune defenses, lice, feces, and chemistries. In that new environment, a microbe must evolve new ways to live and to propagate itself. In several instructive cases doctors or veterinarians have actually been able to observe microbes evolving those new ways.
总之，疾病代表了一步步的演化，而病菌则通过自然选择适应新的宿主和传病媒介。但同牛的身体相比，我们的身体具有不同的免疫系统、虱子、排泄物和化学物质。在这种新的环境下，病菌必须演化出新的生存和传播方法。在几个富有启发性的病例中，医生或兽医实际上已经能够观察到演化出这种新方法的病菌。
The best-studied case involves what happened when myxomatosis hit Australian rabbits. The myxo virus, native to a wild species of Brazilian rabbit, had been observed to cause a lethal epidemic in European domestic rabbits, which are a different species. Hence the virus was intentionally introduced to Australia in 1950 in the hopes of ridding the continent of its plague of European rabbits, foolishly introduced in the nineteenth century. In the first year, myxo produced a gratifying (to Australian farmers) 99.8 percent mortality rate in infected rabbits. Unfortunately for the farmers, the death rate then dropped in the second year to 90 percent and eventually to 25 percent, frustrating hopes of eradicating rabbits completely from Australia. The problem was that the myxo virus evolved to serve its own interests, which differed from ours as well as from those of the rabbits. The virus changed so as to kill fewer rabbits and to permit lethally infected ones to live longer before dying. As a result, a less lethal myxo virus spreads baby viruses to more rabbits than did the original, highly virulent myxo.
得到最充分研究的例子，是多发性黏液瘤病袭击澳大利亚兔子时所发生的情况。这种黏液病毒本来是巴西野兔携带的病毒，据观察，这种病毒在欧洲家兔中造成了一种致命的流行病，而欧洲家兔是另一种不同的兔子。原来，在19世纪有人愚蠢地把欧洲兔引进了澳大利亚，结果造成那里的兔子泛滥成灾。因此，在1950年，黏液病毒被有意识地引进澳大利亚，以期解决这个大陆上的欧洲兔灾。在第一年，黏液病毒在受到感染的兔子中造成了令人满意的(对澳大利亚农民来说)99.8%的死亡率。令这些农民感到失望的是，第二年兔子的死亡率下降到90%，最后下降到25%，使得要在澳大利亚完全消灭兔子的希望落空了。这里的问题是：这种黏液病毒是按照自己的利益来演化的，它的利益不但不同于那些兔子的利益，而且也不同于我们的利益。这种病毒之所以产生变化，是为了少杀死一些兔子，并使那些受到致命感染的兔子多活些时间再死。结果，不那么致命的黏液病毒就能比原先有高度毒力的黏液把下一代病毒传播到更多的兔子中去。
For a similar example in humans, we have only to consider the surprising evolution of syphilis. Today, our two immediate associations to syphilis are genital sores and a very slowly developing disease, leading to the death of many untreated victims only after many years. However, when syphilis was first definitely recorded in Europe in 1495, its pustules often covered the body from the head to the knees, caused flesh to fall off people's faces, and led to death within a few months. By 1546, syphilis had evolved into the disease with the symptoms so well known to us today. Apparently, just as with myxomatosis, those syphilis spirochetes that evolved so as to keep their victims alive for longer were thereby able to transmit their spirochete offspring into more victims.
对于发生在人类中的一个类似的例子，我们只需考虑一下梅毒的令人惊异的演化情况就行了。今天，一提起梅毒，我们立刻会联想到两种情况：生殖器溃疡和病情发展的十分缓慢，许多得不到治疗的患者要过好多年才会死去。然而，当梅毒于1495年首次在欧洲明确见诸记录时，它的脓疱通常从头部到膝部遍布全身，使脸上的肉一块块脱落，不消几个月就使人一命呜呼。到1546年，梅毒已演化成具有我们今天所熟悉的那些症状的疾病。显然，同多发性黏液瘤病一样，为使患者活得长些而进行演化的那些梅毒螺旋体因此就能够把它们的螺旋体后代传染给更多的患者。
THE IMPORTANCE OF lethal microbes in human history is well illustrated by Europeans' conquest and depopulation of the New World. Far more Native Americans died in bed from Eurasian germs than on the battlefield from European guns and swords. Those germs undermined Indian resistance by killing most Indians and their leaders and by sapping the survivors' morale. For instance, in 1519 Cortés landed on the coast of Mexico with 600 Spaniards, to conquer the fiercely militaristic Aztec Empire with a population of many millions. That Cortés reached the Aztec capital of Tenochtitlán, escaped with the loss of “only” two-thirds of his force, and managed to fight his way back to the coast demonstrates both Spanish military advantages and the initial na veté of the Aztecs. But when Cortés's next onslaught came, the Aztecs were no longer naive and fought street by street with the utmost tenacity. What gave the Spaniards a decisive advantage was smallpox, which reached Mexico in 1520 with one infected slave arriving from Spanish Cuba. The resulting epidemic proceeded to kill nearly half of the Aztecs, including Emperor Cuitláhuac. Aztec survivors were demoralized by the mysterious illness that killed Indians and spared Spaniards, as if advertising the Spaniards' invincibility. By 1618, Mexico's initial population of about 20 million had plummeted to about 1.6 million.
人类历史上致命病菌的重要性，可以从欧洲人征服新大陆并使那里人口减少这件事得到很好的例证。印第安人在病床上死于欧亚大陆的病菌的，要比在战场上死于欧洲人的枪炮和刀剑下的多得多。这些病菌杀死了大多数印第安人和他们的领袖，消磨了幸存者的士气，从而削弱了对欧洲人的抵抗。例如，1519年科尔特斯率领600个西班牙人在墨西哥海岸登陆，去征服拥有好几百万人口的勇猛好战的阿兹特克帝国。科尔特斯到达阿兹特克的首都特诺奇提特兰城，又带着他的“仅仅”损失了三分之二的队伍逃走，并一路打回海岸，这既证明了西班牙人的军事优势，也证明了阿兹特克人开始时的幼稚。但当科尔特斯的第二次袭击来到时，阿兹特克人就不再幼稚，而是极其顽强地展开了巷战。使西班牙人取得决定性优势的是天花。1520年，天花随着一个受到感染的来自西班牙属地古巴的奴隶到达墨西哥。由此而产生的天花流行接着杀死了阿兹特克的近一半人口，包括奎特拉瓦克皇帝。大难不死的阿兹特克人也被这种怪病弄得士气低落，因为这种病专杀印第安人而竟不伤害西班牙人，就好像在为西班牙人的不可战胜作宣传似的。到1618年，墨西哥原来2000万左右的人口急剧减少到160万左右。
Pizarro had similarly grim luck when he landed on the coast of Peru in 1531 with 168 men to conquer the Inca Empire of millions. Fortunately for Pizarro and unfortunately for the Incas, smallpox had arrived overland around 1526, killing much of the Inca population, including both the emperor Huayna Capac and his designated successor. As we saw in Chapter 3, the result of the throne's being left vacant was that two other sons of Huayna Capac, Atahuallpa and Huascar, became embroiled in a civil war that Pizarro exploited to conquer the divided Incas.
皮萨罗于1531年率领168个人在秘鲁海岸登陆去征服有几百万人口的印加帝国时，同样带来了一场浩劫。对皮萨罗来说幸运的而对印加人来说不幸的是，天花已在1526年由陆路到达，杀死了印加的很大一部分人口，包括瓦伊纳·卡帕克皇帝和他的指定继承人。我们已在第三章中看到，皇位空缺的结果是使瓦伊纳·卡帕克的另两个儿子阿塔瓦尔帕和瓦斯卡尔卷入了一场内战，使皮萨罗在征服这个分裂的帝国中坐收渔人之利。
When we in the United States think of the most populous New World societies existing in 1492, only those of the Aztecs and the Incas tend to come to our minds. We forget that North America also supported populous Indian societies in the most logical place, the Mississippi Valley, which contains some of our best farmland today. In that case, however, conquistadores contributed nothing directly to the societies' destruction; Eurasian germs, spreading in advance, did everything. When Hernando de Soto became the first European conquistador to march through the southeastern United States, in 1540, he came across Indian town sites abandoned two years earlier because the inhabitants had died in epidemics. These epidemics had been transmitted from coastal Indians infected by Spaniards visiting the coast. The Spaniards' microbes spread to the interior in advance of the Spaniards themselves.
当我们美国人想到存在于1492年的新大陆人口最多的社会时，出现在我们心头的往往只是阿兹特克人和印加人的那些社会。我们忘记了北美洲也曾在那最合逻辑的地方——密西西比河流域养活了人口众多的印第安人社会，我们今天的一些最好的农田就在这个地方。然而，在这种情况下，西班牙征服者对于摧毁这些社会并未起到直接的作用；一切都是由事先已经传播的欧亚大陆的病菌来完成的。当埃尔南多·德索托成为第一个欧洲征服者于1540年在美国东南部行军时，他碰到了两年前因当地居民死于流行病而被放弃的一些城镇旧址。这些流行病是从沿海地区印第安人那儿传来的，而这些印第安人又是被到沿海地区来的西班牙人感染的。西班牙人的这些病菌赶在这些西班牙人之前向内陆传播。
De Soto was still able to see some of the densely populated Indian towns lining the lower Mississippi. After the end of his expedition, it was a long time before Europeans again reached the Mississippi Valley, but Eurasian microbes were now established in North America and kept spreading. By the time of the next appearance of Europeans on the lower Mississippi, that of French settlers in the late 1600s, almost all of those big Indian towns had vanished. Their relics are the great mound sites of the Mississippi Valley. Only recently have we come to realize that many of the mound-building societies were still largely intact when Columbus reached the New World, and that they collapsed (probably as a result of disease) between 1492 and the systematic European exploration of the Mississippi.
德索托仍然看得到密西西比河下游沿岸的一些人口稠密的印第安城镇。在这次远征结束后，又过了很久，欧洲人才又一次到达密西西比河河谷，但这时欧亚大陆的病菌已在北美洲安家落户，并不断向四处传播。到欧洲人下一次在密西西比河下游出现，即17世纪初法国的移民出现时，所有这些印第安人的大城镇已经消失殆尽。它们的遗迹就是密西西比河河谷的那些大土堆。直到最近我们才知道，构筑这种大土堆的社会，有许多在哥伦布到达新大陆时仍然大部分完好无损，它们的瓦解(可能是疾病造成的结果)是从1492年到欧洲人对密西西比河进行系统勘探这一段时间里发生的。
When I was young, American schoolchildren were taught that North America had originally been occupied by only about one million Indians. That low number was useful in justifying the white conquest of what could be viewed as an almost empty continent. However, archaeological excavations, and scrutiny of descriptions left by the very first European explorers on our coasts, now suggest an initial number of around 20 million Indians. For the New World as a whole, the Indian population decline in the century or two following Columbus's arrival is estimated to have been as large as 95 percent.
在我年轻的时候，美国小学生所受到的教育是：北美洲本来只有大约100万印第安人居住。把人数说得这样少，对于为白人的征服行为辩解是有用的，因为他们所征服的只不过是一个可以认为几乎是空无所有的大陆。然而，考古发掘和对最早的欧洲探险者所留下的关于我们海岸地区的描写的仔细研究现已表明，印第安人原来的人数在2000万左右。就整个新大陆来说，据估计在哥伦布来到后的一两个世纪中，印第安人口减少了95%。
The main killers were Old World germs to which Indians had never been exposed, and against which they therefore had neither immune nor genetic resistance. Smallpox, measles, influenza, and typhus competed for top rank among the killers. As if these had not been enough, diphtheria, malaria, mumps, pertussis, plague, tuberculosis, and yellow fever came up close behind. In countless cases, whites were actually there to witness the destruction occurring when the germs arrived. For example, in 1837 the Mandan Indian tribe, with one of the most elaborate cultures in our Great Plains, contracted smallpox from a steamboat traveling up the Missouri River from St. Louis. The population of one Mandan village plummeted from 2,000 to fewer than 40 within a few weeks.
主要的杀手是旧大陆来的病菌。印第安人以前从来没有接触过这些病菌，因此对它们既没有免疫能力，也没有遗传抵抗能力。天花、麻疹、流行性感冒和斑疹伤寒争先恐后地要做杀手的头把交椅。好像这些病还嫌不够似的，紧随其后的还有白喉、疟疾、流行性腮腺炎、百日咳、瘟疫、肺结核和黄热病。在无数情况下，白人实际上在当地亲眼目睹了病菌来到时所产生的破坏。例如，1837年，具有我们大平原最精致的文化之一的曼丹族印第安部落，从一艘自圣路易沿密苏里河逆流而上的轮船上感染了天花。一个曼丹人村庄里的人口在几个星期之内就从2000人急剧减少到不足40人。
WHILE OVER A dozen major infectious diseases of Old World origins became established in the New World, perhaps not a single major killer reached Europe from the Americas. The sole possible exception is syphilis, whose area of origin remains controversial. The one-sidedness of that exchange of germs becomes even more striking when we recall that large, dense human populations are a prerequisite for the evolution of our crowd infectious diseases. If recent reappraisals of the pre-Columbian New World population are correct, it was not far below the contemporary population of Eurasia. Some New World cities like Tenochtitlán were among the world's most populous cities at the time. Why didn't Tenochtitlán have awful germs waiting for the Spaniards
虽然有十几种来自旧大陆的主要传染病在新大陆安家落户，但也许还没有一种主要的致命疾病从美洲来到欧洲。唯一可能的例外是斑疹伤寒，但它的原发地区仍然是有争议的。如果我们还记得稠密的众多人口是我们的群众传染疾病演化的先决条件的话，那么病菌的这种单向交流就甚至更加引人注目。如果最近对前哥伦布时代新大陆人口的重新估计是正确的，它不会比同时代的欧亚大陆人口少得太多。新大陆的一些城市，如特诺奇提特兰城，属于当时世界上人口最多的城市。为什么特诺奇提特兰城没有可怕的病菌在等待着那些西班牙人呢？
One possible contributing factor is that the rise of dense human populations began somewhat later in the New World than in the Old World. Another is that the three most densely populated American centers—the Andes, Mesoamerica, and the Mississippi Valley—never became connected by regular fast trade into one huge breeding ground for microbes, in the way that Europe, North Africa, India, and China became linked in Roman times. Those factors still don't explain, though, why the New World apparently ended up with no lethal crowd epidemics at all. (Tuberculosis DNA has been reported from the mummy of a Peruvian Indian who died 1,000 years ago, but the identification procedure used did not distinguish human tuberculosis from a closely related pathogen (Mycobacterium bovis) that is widespread in wild animals.)
一个可能的起作用的因素是，开始出现稠密人口的时间在新大陆要稍晚于旧大陆。另一个因素是，美洲的3个人口最稠密的中心——安第斯山脉地区、中美洲和密西西比河流域——并未由于经常性的快速贸易而连成一个巨大的病菌繁殖场，就像欧洲、北非、印度和中国在罗马时代连接起来那样。然而，这些因素仍然不能说明为什么新大陆最后显然完全没有任何流行的群众疾病。据报道，在1万年前死去的一个秘鲁印第安人的干尸上发现了肺结核菌的DNA，但在这方面所使用的识别方法并不能把人的肺结核菌同一种亲缘很近的在野生动物中广泛传播的病原体(牛科动物分支杆菌)区别开来。
Instead, what must be the main reason for the failure of lethal crowd epidemics to arise in the Americas becomes clear when we pause to ask a simple question. From what microbes could they conceivably have evolved? We've seen that Eurasian crowd diseases evolved out of diseases of Eurasian herd animals that became domesticated. Whereas many such animals existed in Eurasia, only five animals of any sort became domesticated in the Americas: the turkey in Mexico and the U.S. Southwest, the llama / alpaca and the guinea pig in the Andes, the Muscovy duck in tropical South America, and the dog throughout the Americas.
其实，只要我们暂停一下，问一个简单的问题，那么美洲之所以未能出现流行的致命的群众疾病的主要原因就一定会变得很清楚。这个问题就是，想象一下这些疾病可能会从什么病菌演化而来？我们已经看到，欧亚大陆的群众疾病是从欧亚大陆驯化的群居动物的疾病演化而来的。尽管欧亚大陆有许多这样的动物，但在美洲驯化的动物只有5种：墨西哥和美国西南部的火鸡、安第斯山脉地区的美洲驼／羊驼和豚鼠、热带南美的美洲家鸭和整个美洲的狗。
In turn, we also saw that this extreme paucity of domestic animals in the New World reflects the paucity of wild starting material. About 80 percent of the big wild mammals of the Americas became extinct at the end of the last Ice Age, around 13,000 years ago. The few domesticates that remained to Native Americans were not likely sources of crowd diseases, compared with cows and pigs. Muscovy ducks and turkeys don't live in enormous flocks, and they're not cuddly species (like young lambs) with which we have much physical contact. Guinea pigs may have contributed a trypanosome infection like Chagas' disease or leishmaniasis to our catalog of woes, but that's uncertain. Initially, most surprising is the absence of any human disease derived from llamas (or alpacas), which it's tempting to consider the Andean equivalent of Eurasian livestock. However, llamas had four strikes against them as a source of human pathogens: they were kept in smaller herds than were sheep and goats and pigs; their total numbers were never remotely as large as those of the Eurasian populations of domestic livestock, since llamas never spread beyond the Andes; people don't drink (and get infected by) llama milk; and llamas aren't kept indoors, in close association with people. In contrast, human mothers in the New Guinea highlands often nurse piglets, and pigs as well as cows are frequently kept inside the huts of peasant farmers.
反过来，我们也看到，新大陆驯化动物的这种极端缺乏，反映了用以启动驯化的野生动物的缺乏。在大约13000年前上一次冰期结束时，美洲有大约80%的大型野生哺乳动物便已灭绝了。同牛和猪相比，印第安人剩下的那几种驯化动物不可能成为群众疾病的来源。美洲家鸭和火鸡不是大群在一起生活的，它们也不是我们喜欢搂搂抱抱与我们有大量身体接触的动物(如小绵羊)。豚鼠可能由一种类似恰加斯病或利什曼病的锥虫感染，使我们的一系列痛苦雪上加霜，但这一点还不能肯定。开始，最令人惊奇的是，人类疾病没有一种来自美洲驼(或羊驼)，这使人不由去把这种相当于欧亚大陆牲畜的安第斯山牲畜研究一番。然而，美洲驼有4个方面使它们不能成为人类病原体的一个来源：它们不像绵羊、山羊和猪那样大群饲养；它们的总数绝少会赶上欧亚大陆的家畜种群，因为它们从来没有传播到安第斯山脉以外地区；人们不喝美洲驼的奶(因此不会受到它们的感染)；美洲驼不是在室内饲养，和人的关系不那么密切。相比之下，新几内亚高原地区居民中做母亲的妇女常常用自己的奶喂小猪，而猪也和牛一样经常养在农民的简陋小屋里。
THE HISTORICAL IMPORTANCE of animal-derived diseases extends far beyond the collision of the Old and the New Worlds. Eurasian germs played a key role in decimating native peoples in many other parts of the world, including Pacific islanders, Aboriginal Australians, and the Khoisan peoples (Hottentots and Bushmen) of southern Africa. Cumulative mortalities of these previously unexposed peoples from Eurasian germs ranged from 50 percent to 100 percent. For instance, the Indian population of Hispaniola declined from around 8 million, when Columbus arrived in A.D. 1492, to zero by 1535. Measles reached Fiji with a Fijian chief returning from a visit to Australia in 1875, and proceeded to kill about one-quarter of all Fijians then alive (after most Fijians had already been killed by epidemics beginning with the first European visit, in 1791). Syphilis, gonorrhea, tuberculosis, and influenza arriving with Captain Cook in 1779, followed by a big typhoid epidemic in 1804 and numerous “minor” epidemics, reduced Hawaii's population from around half a million in 1779 to 84,000 in 1853, the year when smallpox finally reached Hawaii and killed around 10,000 of the survivors. These examples could be multiplied almost indefinitely.
源于动物的疾病在历史上的重要性，远远超过了旧大陆与新大陆之间的冲突。欧亚大陆的病菌在大量消灭世界上其他许多地方的土著民族方面起了关键的作用，这些民族包括太平洋诸岛居民、澳大利亚土著居民、非洲南部的科伊桑民族(霍屯督人和布须曼人)。这些以前没有接触过欧亚大陆病菌的民族的累计死亡率在50%和100%之间。例如，伊斯帕尼奥拉岛 [4]的印第安人口，从哥伦布于公元1492年到达时的800万左右减少到1535年的零。麻疹于1875年随着一位访问澳大利亚归来的斐济酋长到达斐济，接着把当时仍然活着的所有斐济人杀死了四分之一(在这之前，大多数斐济人已在1791年死于随着第一批欧洲人的到来而开始的流行病)。梅毒、淋病、肺结核和流行性感冒于1779年随库克船长 [5]到来，接着于1804年又发生了一场斑疹伤寒大流行以及后来的许多“较小的”流行病，把夏威夷的人口从1779年的50万左右减少到1853年的84000人。这一年，天花终于来到了夏威夷，把剩下的人又杀死了1万左右。这种例子多得举不胜举。
However, germs did not act solely to Europeans' advantage. While the New World and Australia did not harbor native epidemic diseases awaiting Europeans, tropical Asia, Africa, Indonesia, and New Guinea certainly did. Malaria throughout the tropical Old World, cholera in tropical Southeast Asia, and yellow fever in tropical Africa were (and still are) the most notorious of the tropical killers. They posed the most serious obstacle to European colonization of the tropics, and they explain why the European colonial partitioning of New Guinea and most of Africa was not accomplished until nearly 400 years after European partitioning of the New World began. Furthermore, once malaria and yellow fever did become transmitted to the Americas by European ship traffic, they emerged as the major impediment to colonization of the New World tropics as well. A familiar example is the role of those two diseases in aborting the French effort, and nearly aborting the ultimately successful American effort, to construct the Panama Canal.
然而，病菌也并不是只对欧洲人有利。虽然新大陆和澳大利亚并没有本土的流行病在等待欧洲人，但热带亚洲、非洲、印度尼西亚和新几内亚却有。旧大陆的整个热带地区的疟疾、热带东南亚的霍乱和热带非洲的黄热病，过去是(现在也仍然是)最著名的热带致命疾病。它们是欧洲人在热带地区殖民的最严重的障碍，同时也说明了为什么直到欧洲人瓜分新大陆开始后将近400年，欧洲人对新几内亚和非洲大部分地区的殖民瓜分才宣告完成。而且，一旦疟疾和黄热病通过船只运输传播到美洲，它们也成了对新大陆殖民的主要障碍。一个为人们所熟知的例子是：这两种病在使法国人修建巴拿马运河的努力中途失败，以及几乎使美国人最后取得成功的修建这条运河的努力中途失败方面所起的作用。
Bearing all these facts in mind, let's try to regain our sense of perspective about the role of germs in answering Yali's question. There is no doubt that Europeans developed a big advantage in weaponry, technology, and political organization over most of the non-European peoples that they conquered. But that advantage alone doesn't fully explain how initially so few European immigrants came to supplant so much of the native population of the Americas and some other parts of the world. That might not have happened without Europe's sinister gift to other continents—the germs evolving from Eurasians' long intimacy with domestic animals.
让我们把所有这些事实牢记心中，在回答耶利的问题时努力重新全面认识病菌所起的作用。毫无疑问，欧洲人在武器、技术和行政组织方面拥有对他们所征服的大多数非欧洲民族的巨大优势。但仅仅这种优势还不能完全说明开始时那么少的欧洲移民是如何取代美洲和世界上其他一些地区那么多的土著的。如果没有欧洲送给其他大陆的不祥礼物——从欧亚大陆人与家畜的长期密切关系中演化出来的病菌，这一切也许是不会发生的。
注释：
1 库鲁病：流行于新几内亚东部的一种致命病毒性脑病，其特征为运动共济失调(如不自主强笑)、战栗样震颤和构音障碍。——译者
2 法罗群岛：或译法俄尔群岛，在北大西洋，曾属丹麦，1948年获得自治。——译者
3 雅司病：指经皮肤接触感染雅司螺旋体而发生的疾病，皮肤损害很像梅毒，主要流行于热带地区。——译者
4 伊斯帕尼奥拉岛：拉丁美洲西印度群岛中部，即海地岛。——译者
5 库克船长：詹姆士·库克(1728—1779)：英国海军上校，航海家，太平洋和南极海洋的探险家。——译者