In his groundbreaking book Guns, Germs, and Steel, biologist Jared Diamond poses the question: Why did Old World diseases devastate the New World and not the other way around? Why did disease move in only one direction?* The answer lies in how the lives of Old World and New World people diverged after that cross-continental migration more than fifteen thousand years ago.
Farming, which allowed people to settle into towns and villages, was independently invented in both the Old World and the New. The key difference was in animal husbandry. In the Old World, a great variety of animals were domesticated, starting with cattle about eight to ten thousand years ago and quickly moving on to pigs, chickens, ducks, goats, and sheep. New World farmers domesticated animals as well, notably llamas, guinea pigs, dogs, and turkeys. But in Europe (and Asia and Africa), the raising and breeding of livestock became a central aspect of life, an essential activity in almost every household. For thousands of years, Europeans lived in close quarters with their livestock and were continuously exposed to their microbes and diseases. In the New World, perhaps because they had more space and fewer domesticated animals, people did not live cheek by fowl with their animals.
Humans do not usually catch infectious diseases from animals; pathogens tend to confine their nasty work to a single species or genus. (Leishmaniasis is a striking exception.) But microbes mutate all the time. Once in a while, an animal pathogen will change in such a way that it suddenly infects a person. When people in the Near East first domesticated cattle from a type of wild ox called an aurochs, a mutation in the cowpox virus allowed it to jump into humans—and smallpox was born. Rinderpest in cattle migrated to people and became measles. Tuberculosis probably originated in cattle, influenza in birds and pigs, whooping cough in pigs or dogs, and malaria in chickens and ducks. The same process goes on today: Ebola probably jumped to humans from bats, while HIV crashed into our species from monkeys and chimpanzees.
Alongside the domestication of animals, humans in the Old World began settling down in villages, towns, and cities. People lived together in much denser numbers than before. Cities, with their bustle, trade, filth, and close quarters, created a marvelous home for pathogens and an ideal staging ground for epidemics. So when diseases migrated from livestock to people, epidemics broke out. Those diseases found plenty of human fuel, racing from town to town and country to country and even crossing the oceans on board ships. Biologists call these “crowd diseases” because that’s exactly what they need to propagate and evolve.
Epidemics periodically swept through European settlements, killing the susceptible and sparing the robust, culling the gene pool. As always, children were the majority of the victims. Almost no disease is 100 percent fatal: Some victims always survive. The survivors tended to have genes that helped them resist the disease a little better, and they passed that resistance on to their children. Over thousands of years and countless deaths, people in the Old World gradually built up a genetic resistance to many brutal epidemic diseases.
In the New World, on the other hand, no big-time diseases seem to have leapt from animals into the human population. While the Americas had cities as large as those in Europe, those cities were much newer at the time the Spanish arrived. People in the New World hadn’t been living in close quarters long enough for crowd diseases to spring up and propagate. Native Americans never had the opportunity to develop resistance to the myriad diseases that plagued Europeans.
This genetic resistance, by the way, should not be confused with acquired immunity. Acquired immunity is when a body gets rid of a pathogen and afterward maintains a state of high alert for that same microbe. It’s why people don’t normally get the same illness twice. Genetic resistance is something deeper and more mysterious. It is not acquired through exposure—you are born with it. Some people are born with greater resistance to certain diseases than others. The experience of our team in the valley of T1 is a textbook illustration. The doctors believe everyone on the expedition was bitten and exposed. Only half, however, came down with the disease. A few, like Juan Carlos, were able to fight it off without drugs. Others became seriously ill and some, even as I write this, are still struggling with the disease.
The genes that resist disease can only spread in a population through the pitiless lottery of natural selection. People with weaker immune systems (children especially) must die, while the stronger live, in order for a population to gain widespread resistance. A staggering amount of suffering and death over thousands of years went into building European (and African and Asian) resistance to crowd diseases. One biologist told me that what probably saved many indigenous Indian cultures from complete extinction were the mass rapes of native women by European men; many of the babies from those rapes inherited European genetic resistance to disease. (The scientist, after telling me this horrifying theory, said, “For God’s sake don’t attach my name to that idea.”)
In the New World, these many thousands of years of anguish and death were compressed into a window from 1494 to around 1650. The mass murder by pathogen happened in that one cruel century and a half, and it struck at precisely the worst moment, when the population of the New World had recently coalesced into big cities and reached the levels of density necessary for those epidemics to spread furiously. It was a perfect storm of infection.
We do not hear many of the voices of these victims. Only a handful of Native American eyewitness accounts of the cataclysm survive. One in particular stands out, a remarkable text called the Annals of the Cakchiquels, which describes an epidemic, probably smallpox or flu, that swept an area in Guatemala northwest of Mosquitia. This extraordinary manuscript, discovered in a remote convent in 1844, was written in a Mayan language called Cakchiquel by an Indian named Francisco Hernández Arana Xajilá. As a teenager, Arana Xajilá lived through the epidemic that destroyed his people.
It happened that during the twenty-fifth year [1520] the plague began, oh my sons! First they became ill of a cough, they suffered from nosebleeds and illness of the bladder. It was truly terrible, the number of dead there were in that period. The prince Vakaki Ahmak died then. Little by little heavy shadows and black night enveloped our fathers and grandfathers and us also, oh my sons!… Great was the stench of the dead. After our fathers and grandfathers succumbed, half of the people fled to the fields. The dogs and the vultures devoured the bodies. The mortality was terrible… So it was that we became orphans, oh my sons! So we became when we were young. All of us were thus. We were born to die!