Pestilence has molded both our cultures and our genes over the long term. But before tackling these long-term effects, let’s look at what actually happened to human civilizations when diseases struck. Over time, humans increased in numbers and gathered together in villages, towns, and then cities. Populations grew larger and denser as civilization progressed. Sooner or later, when people are crowded together, infectious disease takes the opportunity to spread itself around, too.

Let’s focus on severe infections with high mortality rates (such as smallpox, Ebolavirus, or bubonic plague). Each time pestilence passes through society, a sizable fraction is wiped out. The survivors give birth to the next generation, and numbers gradually increase again. Once the population is dense enough, another epidemic strikes and the cycle repeats. Over many generations, genetic resistance develops, so virulent pestilences wane into childhood diseases and may eventually fade away completely. Meanwhile, novel infections emerge and spread, taking the place of yesterday’s retired plagues. A crucial point for understanding the long-term effects of an epidemic is that death is distributed neither equally nor at random. Infectious disease strikes harder at some segments of the population than others. Let’s start with factors that are wholly or mostly biological in nature.

First, older people and young children are especially susceptible. This is because the human immune system is not fully developed in the very young and is beginning to fade away in the very old. Conversely, some individuals may be immune to certain infections. Nowadays, such immunity is mostly due to artificial vaccination. Before this was available, immunity was normally acquired the hard way by catching the disease and surviving.

Second is the phenomenon of genetic resistance to disease. In contrast to immunity, which is acquired during an individual’s lifetime, genetically based resistance is inherited from one’s ancestors. Individual people differ greatly in their inherent susceptibility to different diseases. When a population suffers from a dangerous disease, those with genetically based resistance survive more often than others. Consequently, inherited resistance gradually builds up over several generations. For example, the earliest reliable accounts of smallpox in Asia and Europe suggest that it was fatal 75% or more of the time. Yet over the next thousand years, the mortality rate fell to around 10%–30%. Then when smallpox was carried to the New World, the mortality rate among the American Indians was 75% or more. Thus, a virulent disease is vastly more devastating in a population that has never been previously exposed and has had no opportunity to build up resistance.

Many other factors affect our susceptibility to disease. These range from mainly biological to largely social in nature. For example, those who are poorly fed or live in bad housing and are cold, wet, and dirty are much more at risk than well fed people who are dry, warm, and clean. Obviously, the closer people are crowded together, the easier it is for infectious disease to spread. These factors all lie on the interface where social conditions merge with biological effects.

Many of these factors can be lumped together, at least crudely. To put it bluntly, poor people are both more likely to become infected and also more likely to die if they are infected. Unfair as it may be, this is inevitably true in all real- life human societies. Today this is most clearly seen in the contrast between the industrial nations and the Third World. However, throughout recorded history prosperity and status have had their advantages. Even in societies of apes and baboons, higher-status animals tend to be better fed, a factor that helps them fight off many infections.

But how do you get to be rich or poor, high or low? To be sure, one way is to inherit money or social position (as distinct from genes) from your parents. But this ignores how your ancestors got rich to start with. Do competent, industrious, attractive, healthy, and brave people tend to rise through the ranks of society? Or is it the ruthless, greedy, cowardly, and corrupt who claw their way to the top? Whichever of these alternatives you espouse, for better or for worse, during a virulent epidemic, fewer people at the top will die than those at the bottom.