So far, Diamond has addressed how differences in environment led to differences in food production, which spread around the world at different rates. Diamond will now show how agricultural differences between civilizations led to vast differences in literacy, health, technology, and government.
The differences between agricultural societies and hunter-gatherer societies are the most basic differences that arise between human beings because of the influence of geography. Yet according to Diamond, these original differences could then be said to “snowball” into larger differences, as we will see in the chapters to come.
Diamond remembers meeting a farmer who attempted to have sex with a sheep and contracted a horrible disease from the animal. While there are few people who would try to imitate the farmer, the fact remains that people live in close proximity to animals, and catch diseases from their animals.
The point of this rather disgusting anecdote is that if given enough time in close contact with animals, people catch diseases from them in all kinds of ways, sexual and otherwise.
Germs and viruses have been some of the leading causes of death in human history. Many of the key plagues and epidemics of history—the Black Death, the Spanish Flu—have involved diseases that spread from animals to human beings. Consider this from the microbes’ point of view. The most “successful’ microbes, which have evolved over millions of years, are those which can pass from host to host quickly and efficiently. Microbes have evolved to move through saliva and other bodily fluids, and to provoke reactions like bleeding, vomiting, open sores, etc.—anything that passes on the microbes to other hosts (even if it kills them).
The first major point of this chapter is that diseases survive within “hosts” (many of which are human bodies). Furthermore, the most successful diseases reproduce themselves quickly inside their hosts, so that they never entirely die out. The symptoms of a disease tend to spread the disease—coughing, sneezing, etc.—because, according to the process of natural selection, the most successful viruses and germs have been those that provoke such responses in their hosts so that they (the viruses and germs) can reproduce.
Humans have a few different defenses against germs, such as coughing, sneezing, running a fever, etc. Another important “defense” against disease is evolution itself: over the centuries, human beings with weak immune systems are more likely to die off without having children, eliminating themselves from the gene pool. So over time, humans have evolved to be immune to diseases—which leads diseases to evolve to become more infectious, and so on.
Over time, populations die out or else develop immunities to diseases. This means that the diseases themselves either die out or mutate into new, more dangerous diseases. As with any other aspect of natural selection and the “food chain,” the diseases and their hosts are constantly evolving to outdo one another.
A small human community, such as a community of hunter-gatherers, might suffer from any number of lethal diseases. But because of its small size, the community would either 1) die off quickly, or 2) survive, with everyone in the community developing immunity to the disease, in which case the disease would die off. There are certain diseases that could only survive in a group of many people—for example, measles can only survive in a population of at least half a million people. In such “crowd diseases,” the microbe needs many human hosts to survive: it needs humans to infect, plus a perpetual “crop” of children to infect after it’s killed off everyone else in the community (or after everyone else has caught the disease, survived, and developed an immunity to it).
One of the most important factors in determining whether or not a community will die of an epidemic is population size. A small community (i.e., a hunter-gatherer community) can easily be wiped out altogether by disease. Larger communities, however, will contain some people with immunities to the disease, ensuring that the community as a whole survives, even if a majority of it dies—and those who do survive will usually pass on their immunity to their offspring. On the other hand, there are some diseases that only exist in large communities—meaning that those large communities are the only groups that develop immunities to the diseases (so that both the disease and its host—the community itself—survives). This in turn means that when a large community interacts with a smaller community, a greater portion of the small community than of the (partially immune) large community will die of the disease.
The rise of agriculture coincided with the rise of crowd diseases. Farming communities were bigger, denser, and more sedentary, and everyone shared resources like food and water. The rise of cities was also important for the spread of crowd disease, because cities allowed for sufficient numbers of people living close to one another.
Agriculture leads to the emergence of more widespread deadly diseases because people are close together, and also close to animals—recall the relationship Diamond discussed between agriculture and population density in Part Two.
Another key reason that crowd diseases first appeared among agriculturalists was the presence of domesticated animals. Animals carry huge numbers of microbes, and most deadly human diseases began as diseases of animals. For example, the AIDS virus was once a disease of monkeys. So agriculturalists will be more likely to suffer from infectious diseases—but because of their large populations, their communities will also be more likely to survive the diseases.
Many animals spread disease, meaning that agricultural societies in which people are often around animals will tend to have more diseases—but also develop more immunities.
The European explorers who came to the New World brought diseases like smallpox with them: these diseases killed huge numbers of Native Americans, whose immune systems had no defenses against them. One reason the Native Americans had no natural defenses against smallpox and other diseases was that they hadn’t interacted with domesticated animals. On the other hand, Europeans hadn’t interacted with certain diseases of the New World, such as yellow fever and malaria, and many European explorers died from such diseases. Nevertheless, infectious diseases acted as a net benefit to the Europeans when they colonized the New World, because, by and large, the Europeans had stronger immune systems.
When a large, agricultural society like early modern Spain encounters a small, hunter-gatherer society like the 16th century Native Americans, the latter society will die of the larger society’s diseases. By and large, the Europeans who colonized the New World had immunities to the same diseases that proved so lethal to the Native Americans—a result, Diamond argues, of their historical membership in dense, agricultural societies.