Chapter 12 Chapter 11 The Deadly Gift of Livestock
We have now looked at how food production arose in several centers and how it spread from there to other areas at different rates.These geographic differences are the final, important answer to Yali's question about why different peoples end up with vastly different levels of power and affluence.However, food production itself is not the direct cause.In a one-on-one fight, an unarmed farmer might not be a match for an unarmed hunter-gatherer.
Part of the explanation for farmers' power lies in the much denser populations that food production could support: 10 unarmed farmers could certainly outmaneuver one unarmed hunter-gatherer in battle.Another part of the explanation: Neither farmers nor hunter-gatherers were unarmed, at least not in a metaphorical sense.Peasants tend to breathe out scarier germs, have better weapons and armor, master generally more effective technology, and live under centralized governments that know better how to wage wars of conquest An outstanding figure of cultural accomplishment.The next four chapters, therefore, explore how the ultimate cause of food production leads to the immediate causes of germs, literacy, technology, and centralized government.
The relationship between livestock, crops, and germs was memorably illustrated to me by a hospital case that I heard from a doctor friend.When my friend was a young doctor, he was called into a hospital room to see a couple afflicted by a strange disease.The couple had trouble communicating with each other, and with my friend, which was not good for healing.The husband was a timid little man, who had pneumonia from some germ.He could speak only a few words of English, and his beautiful wife acted as an interpreter.She was worried about her husband's illness and was terrified of the unfamiliar hospital environment.My friend was exhausted after working in the hospital for a whole week trying to figure out what unusual risk factors were causing this strange disease.Physical and mental exhaustion made him forget all that he had been taught about the patient's secret: he made the terrible mistake of asking the wife to ask her husband if he had had any sexual experiences that might have caused the infection.
The doctor noticed that the husband became flushed and curled up, making his thin frame seem even smaller.He longed to get under the sheets and stammer a few words in a barely audible voice.His wife suddenly let out an angry cry, stood up straight, and faced him condescendingly.Before the doctor could stop him, she grabbed a heavy metal bottle, threw it at her husband's head with all her strength, and then ran out of the room angrily.It took the doctor a lot of work to wake him up, and even more work to pick out from the man's broken English what he had done to make his wife so furious.The answer came slowly: it turned out that he had just admitted that he had sex with ewes a few times during a visit to the family farm not long ago, and that perhaps this was what had infected him with the mysterious germ.
The incident sounded absurd, and could have no more general significance.But in fact, it illustrates a very important big problem: human diseases of animal origin.Our love for sheep is seldom as carnal as the patient's.But most of us have a platonic love for our pets, cats, dogs, and the like.Judging by the sheer number of sheep and other livestock we keep.Our society undoubtedly has a passing fondness for them.For example, a recent census shows that Australians attach great importance to their sheep, with 17.0854 million people raising 1.616 sheep.
Some of us adults get infections from our pets, and children get them even more.Often the illness is nothing more than a minor discomfort, but some can develop into major illnesses.The leading killers of humans throughout recent history have been smallpox, influenza, tuberculosis, malaria, plague, measles, and cholera, all infectious diseases that have evolved from diseases of animals, although oddly enough the germs that cause most of our epidemics in humans Today it is almost exclusively restricted to humans.Before World War II, more war victims died from war-induced disease than from combat trauma.All the military history that glorifies great generals understates the depressing fact that the victors in past wars were not always the armies with the best generals and best weapons, And often it's just the ones that carry the scariest germs that can be passed on to sick people.
The most forbidding example of the role of germs in history.From the European conquest of the Americas that began with Columbus' voyage in 1942.Although countless Indians were killed by the murderous Spanish conquistadors, far more were killed by the vicious Spanish germs.Why was there such an unequal exchange of this dreadful germ between Europe and America?Why didn't Indian diseases kill the Spanish invaders in large numbers and spread back to Europe, wiping out 95% of Europe's population?Similar problems arose not only in the mass killing of Europeans who would-be conquerors of the African and tropical Asia, but also in the mass killing of many other natives by Eurasian germs.
The problem of animal origin of human disease is thus an underlying cause of the broadest patterns in human history and of some of the most important problems in human health today. (Think of AIDS, a very fast-spreading human disease that seems to have evolved from a virus in African wild monkeys.) This chapter begins by considering what a "disease" is, asking why some germs Evolution's purpose is to "make us sick", and most other organisms don't make us sick.We will examine why many of the most familiar infectious diseases become epidemics and spread rapidly, such as the current AIDS epidemic and the medieval epidemic of the bubonic plague (bubonic plague).We will then consider how the ancestors of the germs that now circulate only among us transferred to us from their original host animals.Finally, we'll look at how insight into the animal origin of our infectious diseases helps explain the significant, almost one-way exchange of germs between Europeans and Indians.
Naturally, we all like to think about disease in our own light: How can we save ourselves and kill the pathogen?Let's destroy these villains, no matter what their motives are!However, in ordinary life, in order to defeat the enemy, one must know the enemy, and this is especially true in medicine.
So let us first set aside our human prejudices for a moment and consider disease from the perspective of germs, which, like us, are the product of natural selection.Germs make us sick in all sorts of weird ways, like giving us genital sores or diarrhea.What evolutionary benefit would it gain by doing so?This seems a particularly puzzling and self-defeating thing to do, because when a germ kills its host it also kills itself.
Fundamentally, germ fireworks are like any other species.Evolution selects for those individuals that are the most effective at both producing offspring and helping them disperse to suitable habitats.The spread of a germ can be defined mathematically as the number of new victims infected by each original patient.The size of this number depends on how long each victim is able to infect new victims, and how efficiently the germ is transferred from one victim to the next.
Germs have evolved various ways to spread from one person to another, and from animals to humans.Bacteria with strong transmissibility have more offspring, and as a result, they will be favored by natural selection.Many of the "symptoms" of our diseases are really nothing more than ways some very clever germs can change our bodies or behavior so that we help spread them.
The least effortful way for germs to spread is to wait to pass it on to the next victim.Some pathogens wait for the next host to be eaten by the next host, which is the use of this strategy: for example, salmonella is infected because we eat infected eggs or meat; Pigs enter us from eating undercooked pork after killing them; the parasite that causes nematode granulosis is something sushi-loving Japanese and Americans sometimes get from eating sashimi.These parasites are passed to humans from animals that are eaten, but the virus that causes laughing sickness in the New Guinea highlands is usually passed from one person to another.The disease is spread through cannibalism: mothers in the highlands dug out the brains of people who died of kuru disease and waited for cooking, and the children beside them fiddled with the uncooked brains and then added their fingers. Thus committed a fatal mistake.
Instead of waiting until the old host dies to be eaten, some germs "hitch a ride" in the saliva of an insect that bites the old host and then sprints off to find a new host.Providing this “free ride” might be mosquitoes, fleas, lice or tsetse flies, which transmit malaria, plague, typhoid or sleeping sickness, respectively.The most despicable trick of this passive transmission is through the women to the children, thereby infecting the babies after birth.The pathogens that cause syphilis, rubella, and now AIDS play such tricks to create moral dilemmas against which those who advocate a world that is basically right have to fight to the death.
Other germs can be said to take care of things themselves.They change the structure and habits of their hosts to speed up their spread.From our point of view, having exposed genital sores caused by an STD like syphilis is a deeply disgraceful thing.From the germ's point of view, however, they are merely a useful means of enlisting the host's help to colonize the body cavity of a new host.Skin lesions caused by smallpox are also transmitted by direct or indirect physical contact (sometimes quite indirect, as in the case of white Americans bent on exterminating "aggressive" Indians who used blankets previously used by smallpox patients as gift to them).
Influenza, common cold, and pertussis bacteria employ even more powerful strategies, coaxing their victims to cough or sneeze, spraying a swarm of germs toward a new prospective host.Likewise, the cholera bacterium induces profuse diarrhea in its victims, sending the germs into the drinking water of potential new victims.The virus that causes Korean hemorrhagic fever is spread through rat urine.Nothing compares to the rabies virus in altering the behavior of its host, which not only enters the saliva of an infected dog, but drives the dog into a frenzy of biting that leaves many new victims are infected.But parasites like hookworms and schistosomes deserve the prize for the little bug's actual efforts.Their larvae are excreted in the faeces of a previous victim into water or soil, from where they work their way into the skin of their new host.
So, from our point of view, genital ulcers, diarrhea, and coughing are all "symptoms."But from the germ's point of view, they're clever evolutionary strategies for spreading germs.That's why it's in the germ's interest to "make us sick".But why did germs evolve such an apparently self-defeating strategy of killing their hosts?
From the germ's point of view, that's just an unintentional by-product of host symptoms facilitating the efficient spread of the germ (what a great consolation to us!).Yes, an untreated cholera patient could end up dying of gallons of diarrhea a day.However, at least for a while, as long as the patient is alive, the cholera bacteria benefit by spreading in large numbers into the drinking water sources of the next victim.If each victim thus infects on average more than one new victim, the cholera germs will still spread even if the first host happens to die.
So much for our dispassionate examination of the interests of germs.Now, let's step back and consider our own selfish interests: the best way to stay alive and healthy is to kill those damn germs.A common reaction we have to an infection is a fever.And once again we are used to treating fever as a "symptom", as if it just happened for no reason.However, the regulation of body temperature is controlled by genes and does not happen for no reason.Some diseases respond to heat more sensitively than our bodies.Raising our body temperature is essentially roasting germs before we roast ourselves.
Another common response we have is to mobilize our immune system.Our white blood cells and other cells actively seek out and kill foreign germs.The specific antibodies we develop to fight off a germ that infected us make it less likely that we'll get infected again after we've recovered.We all know from experience that with some diseases, such as influenza and the common cold, our resistance to them is only temporary;However, against other diseases—including measles, mumps, rubella, pertussis, and now smallpox—our antibodies elicited by a single infection confer lifelong immunity.That's how vaccination works: inoculating us with a dead or weakened strain of bacteria triggers our antibodies without actually getting sick.
However, some clever germs just won't succumb to our immune defenses.Some have learned to fool us by changing certain molecular structures of the bacteria that our antibodies recognize (the so-called antigens).New strains of influenza are constantly evolving or retooling to produce different antigens, which is why even though you had the flu two years ago, you are not immune to infection when another flu arrives this year.Malaria and sleeping sickness are even harder customers to catch because of their ability to rapidly change antigens.The hardest thing to catch is AIDS, because it can evolve new antigens even in a patient, destroying the patient's immune system.
Our slowest defensive responses are exhibited by natural selection.Natural selection has altered the frequencies of our genes from generation to generation.For almost any disease, some people prove to be more genetically resistant than others.During an epidemic, those who have a gene that resists a certain germ are more likely to survive than those who lack the gene.Thus, over the course of history, the proportion of individuals with those resistance genes among populations repeatedly exposed to a pathogen is higher—simply because unlucky individuals without the genes are less likely to survive Their genes are passed on to offspring.
Once again, you may think that this is a great relief.This evolutionary response does no good to genetically susceptible dying individuals.However, it does mean that the population as a whole has a greater ability to resist the pathogen.Examples of such genetic defenses are the sickle cell gene, the Tessa gene, and the cystic fibrosis gene that may enable black Africans, Ashkenazi Jews, and Nordics, respectively, to protect themselves (at some cost) against malaria , tuberculosis and bacterial diarrhea.
In short, our interactions with most species, as our relationship with hummingbirds demonstrates, do not make us "sick," nor do hummingbirds "sick."Neither we nor the hummingbirds needed to evolve the ability to guard against each other.This peaceful relationship is maintained because hummingbirds do not expect us to reproduce for them or to give them our bodies for food.Hummingbirds evolved to feed on nectar and insects, which they acquired through the use of their wings.
But germs have evolved to feed on the nutrients inside us, and they don't have the wings to fly to a new victim once the original victim dies or develops resistance.As a result, many germs have had to evolve tricks to allow them to spread among potential victims, and many of these tricks are the "symptoms" that we experience.We have also evolved our own anti-tricks, to which bacteria have evolved anti-anti-tricks in response.We and our pathogenic manifestations are locked in an escalating evolutionary contest, at the cost of the death of one of the contestants, in which natural selection is the judge.Let me now consider the format of this contest: Is it blitzkrieg or guerrilla warfare?
Suppose we count the number of cases of a certain infectious disease in a certain area and note how these numbers change over time.The resulting patterns of change vary widely across diseases.For some diseases, such as malaria or hookworm, new cases can appear in any month of any year in an affected area.However, a so-called epidemic may have no cases for a long time, then a large number of cases, and then no cases for a while.
Of these epidemics, influenza is the one most Americans are very familiar with from experience, and some years have been particularly bad for us (but good years for the influenza virus) .Epidemics of cholera occur at longer intervals, and the 1991 Peruvian cholera epidemic was the first to reach the New World in the 20th century.While influenza and cholera epidemics make headlines today, epidemics before the advent of modern medicine were often far scarier.The single largest epidemic in human history was influenza, which killed 21 million people at the end of World War I.The Black Death (bubonic plague) killed a quarter of Europe's population between 1346 and 1352, in some cities as much as 70%. When the Canadian Pacific Railway passed through Saskatchewan in the early 1880s, a staggering nine per cent of the annual deaths from tuberculosis among Indians in the province, who had had little previous exposure to whites and their germs, occurred.
These infectious diseases that visit us as epidemics rather than as adjuncts have several common characteristics.First, they spread rapidly and efficiently from one infected person to nearby healthy people, with the result that entire populations become infected within a very short period of time.Second, they are "acute" illnesses: within a short period of time, you either die or fully recover.And third, the lucky ones among us who do recover develop antibodies that allow us to live without fear of a relapse for a long time, possibly a lifetime.Finally, these diseases are often only transmitted in humans; the germs that cause them often do not live in the soil or on other animals.All four of these characteristics also apply to what Americans consider common childhood acute infectious diseases, including measles, rubella, mumps, whooping cough, and smallpox.
The combination of these four characteristics often results in the prevalence of certain diseases, and the reasons are not difficult to understand.Simply put, this is what happened: the rapid spread of the germ and the rapid development of symptoms meant that everyone in the local population was quickly infected and either died or recovered and became immune shortly thereafter.None of the people who would still be infected would survive.But since the germ cannot survive except in a living human body, the disease disappears after death, until another batch of descendants reach the age of susceptibility—until an infected outsider makes a The epidemic restarted.
A typical example of how these diseases became epidemics is the history of measles on isolated islands in the Atlantic Ocean called . A severe measles epidemic reached the Faroe Islands in 1781, then disappeared, and the islands were free of measles until 1846, when an infected carpenter arrived by ship from Denmark.Within three months, almost the entire population of the Faroe Islands (7,782 people) had measles, some died, some recovered, and the measles virus disappeared again until the next epidemic.Some studies suggest that measles may disappear in any population of fewer than 500,000 people.Only in relatively large populations does the disease move from one area to another, until enough babies are born in the previously infected area that measles returns.
This applies to the case of measles in the Faroe Islands, but also to other acute infectious diseases with which we are familiar around the world.In order to sustain themselves, these diseases require a large enough population, sufficiently crowded and densely populated, that by the time the disease would otherwise decline, there would be a large pool of susceptible children ready to infect.Therefore, measles and some similar diseases are also called mass diseases.
Clearly, mass disease cannot exist among small hunter-gatherer groups and slash-and-burn farmers.The tragic experience of modern Amazonian Indians and Pacific Islanders shows that a small tribe can be nearly wiped out by an epidemic brought by an outsider -- because none of the small tribe has any resistance to the disease. Antibody.For example, in the winter of 1902, a dysentery epidemic brought by a sailor on the whaling ship Active killed 51 of 56 Sadlermiut Eskimos living in Southampton in the Canadian Arctic. A group of completely isolated people on the island of Ton.Furthermore, measles, and some of our other "childhood" diseases, are more likely to kill infected adults than children, and the adults in that small tribe are all susceptible. (Measles is rare among modern American adults, by contrast, because most of them either had measles or were vaccinated as children.) Most kill and then disappear.Small tribes have small populations, which not only explains why they cannot withstand epidemics brought in from outside, but also why they have not been able to evolve their own epidemics to pay back to the outsiders.
However, this does not mean that a small population is immune to all diseases.They also get infectious diseases, but only a few kinds of infectious diseases.Some infectious diseases are caused by germs that can live on animals or in the soil. As a result, the disease will not disappear and people will always be infected.For example, the yellow fever virus is carried by wild monkeys in Africa, and it has always been able to infect rural populations in Africa through wild monkeys, from which it was carried by the transatlantic slave trade to infect monkeys and people in the New World.
There are also some infectious diseases that occur in sparsely populated places, and they are chronic diseases such as leprosy and the like.Since the disease can take a long time to kill its sufferer, the sufferer, while alive, becomes a reservoir of germs that infect other members of the small tribe.For example, I worked in the 1960s in the highlands of New Guinea in Karimuibasim, an isolated population of several thousand who had one of the highest rates of leprosy in the world—about 40 %!After all, groups with small populations are prone to some non-fatal infectious diseases.Since we have not developed immunity to this infectious disease, it turns out that the same person can get reinfected after recovering.This is the case with hookworms and many other parasites.
All these diseases, peculiar to isolated and small populations, must be among the oldest of the human race.They are diseases that we were able to develop and sustain in the early millions of years of our evolutionary history, when the overall population was small and scattered.These are diseases we share with, or have diseases similar to, our wild relatives, the African great apes.In contrast, mass disease of the kind we have been discussing is only possible when a large, dense population accumulates.This accumulation of populations began with the beginning of agriculture about 10,000 years ago, and then accelerated with the beginning of cities thousands of years ago.In fact, the earliest attested dates for many familiar infectious diseases are surprisingly late: smallpox around 1600 BC (inferred from pockmarks on an Egyptian mummy), parotid Inflammation appeared in 400 BC, leprosy in 200 BC, polio in 1840 AD, and AIDS in 1959.
Why should the advent of agriculture be the beginning of our mass contagion?One of the reasons, mentioned earlier, is that agriculture maintained a much higher population density than hunter-gatherers—10 to 100 times higher on average.In addition, hunter-gatherers often change camps, leaving behind piles of excrement that collect a large number of germs and parasite larvae.But farmers are sedentary, living in their own sewage, providing an easy path for germs to get from one person's body to another's drinking water source.
Some agricultural populations collect their own manure and spread it as fertilizer on the fields where people work, making it even easier for the germs and parasites in the manure to infect new victims.Irrigated agriculture and fish farming provide ideal living conditions for snails and leeches.Snails host blood-sucking worms, while leeches burrow into our skin as we wade through fecal-laden water.Settled farmers were surrounded not only by their own feces but by disease-spreading rodents attracted by their stored grain.Forest clearings cleared by African farmers also provide ideal breeding grounds for malaria mosquitoes.
If the advent of agriculture has thus blessed our germs, the advent of cities has blessed them even more, for denser populations aggravate the situation with even worse sanitation. up.It was not until the early 20th century that Europe's urban population finally stabilized: until then, the steady migration of healthy peasants from the countryside to the cities was necessary to replace those dying of mass disease in the cities.Another good thing for germs was the development of world trade routes that, by Roman times, effectively linked Europe, Asia, and North Africa into one gigantic germ breeding ground.It was around this time that smallpox, the so-called Antonine plague, finally reached Rome, killing millions of Roman townspeople between AD 165 and 180.
Likewise, the bubonic plague, the so-called Plague of Justinian, appeared for the first time in Europe (A.D. 542-543).But it wasn't until AD 1346 that the so-called bubonic plague began to hit Europe in full force, when a new overland trade route with China provided flea-infested furs with a path along the east-west axis of Eurasia, from everywhere plague The fast transportation channel from Central Asia to Europe.Today, our jets make even the longest intercontinental flight shorter than the duration of any infectious disease in humans.This is how in 1991 an Aerolineas Argentinas plane parked in Lima (Peru) managed to fly over 3,000 miles from Lima to deliver dozens of cholera-infected people that same day to Los Angeles, the city where I live.The rapid increase in American travel and foreign immigration to the United States is turning us into another melting pot—this time of germs that we thought were causing weird diseases in faraway lands but never Take it for granted.
When, therefore, the population has reached a certain level of number and concentration, we have reached a stage in history in which we are at least able to form and maintain the mass diseases that only we humans have.But this conclusion also has its paradox: it was impossible for these diseases to exist before then!Instead, they must evolve into new diseases.So where do these new diseases come from?
More recently, evidence is emerging thanks to molecular studies of pathogenic bacteria.For many of the germs that cause diseases unique to us, molecular biologists are now able to identify some of the closest relatives.These germs have also proven to be vectors of mass contagion—but only among our various domestic animals and pets!In animals, too, epidemics require large, dense populations, rather than afflicting any one animal: these epidemics occur mainly only in social animals that require large populations.So when we domesticated social animals like cattle and pigs, they were already suffering from some epidemic disease just waiting to be transferred to us.
For example, the measles virus is most closely related to the rinderpest virus.This dire epidemic affects cattle and many wild ruminant mammals, but not humans.Measles, in turn, does not attack cattle.The close resemblance between measles virus and rinderpest virus suggests that the latter migrated from cattle to humans and then evolved into measles virus by changing its properties to suit humans.Considering that many farmers lived and slept so close to their cows and their dung, breath, sores and blood, this shift was not at all surprising.Our close relationship with cattle has existed for 9,000 years, since we domesticated them—a lot of time for the rinderpest virus to discover that we are in its vicinity.As Table 11.1 shows, several other infectious diseases with which we are familiar can also be traced to illnesses in our animal friends.
Given our close relationship with our favorite animals, we must be constantly attacked by their germs.These invaders are sifted by natural selection, and only a few make it to human diseases.As long as we take a quick look at some current diseases, we can see the four stages of evolution from animal diseases to human diseases.
The first stage is exemplified by the dozens of diseases we sometimes acquire directly from our pets and domestic animals.They include cat scratch fever from our cats, leptospirosis from our dogs, psittacosis from our chickens and parrots, and cloth from our cattle Rucellosis.We also pick up diseases from wild animals, for example a hunter might get tularemia while skinning a hare.All of these germs are still in the early stages of their evolution into human-transforming pathogens.They still cannot be transmitted directly from one person to another, and even their transfer from animals to us is still rare.
In the second stage, the evolution of the original animal pathogen has reached the point where it can be transmitted directly in the human population to cause an epidemic.However, this epidemic disappeared due to several reasons, such as being cured by modern medicine, or because everyone around has already been sick, some gained immunity, and some died.For example, there used to be an unknown fever called Oneon-Nyonne fever that emerged in Africa in 1959 and went on to infect millions of Africans.It was presumably caused by a virus in monkeys that was transmitted to humans by mosquitoes.The fact that the patient recovers quickly and does not relapse contributes to the rapid disappearance of this emerging disease.A disease in America's homeland is Fort Brigge fever, the name given to an emerging leptospirosis that broke out in the United States in the summer of 1942 and soon disappeared.
A deadly disease that disappeared for another reason was New Guinea's kuru.It is caused by cannibalism and is caused by a slow-acting virus. Once a person is infected with this virus, he will not recover for life.Just as kuru was on the verge of wiping out the Fore tribe of 20,000 in New Guinea, the Australian government established governance of the area around 1959, ending the practice of cannibalism and thus the spread of kuru .The history of medicine is full of ailments that are unheard of today, but which once caused terrifying epidemics and then disappeared as mysteriously as they appeared.There are many epidemics that disappeared long before modern medicine developed the means to identify the culprit bacteria, the "English sweating fever" that spread rapidly and alarmed Europe from 1485 to 1552, and 18, The "Picardy sweat fever" in France in the 19th century is just two of them.
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，这种螺旋体就从口丁咬处进人人体。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）。
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。
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。
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。
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％的死亡翠。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。
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市全身，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岗。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。
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。
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。
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。
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邵时，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。
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％。
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。
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？
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。
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。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13，000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）。豚鼠可能有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。
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，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。
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。
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——从欧亚大陆人和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。
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