I

The Pitiless War

Infectious disease is one of the great tragedies of living things—the struggle for existence between different forms of life. . . . Incessantly the pitiless war goes on, without quarter or armistice.

—Hans Zinsser, Rats, Lice and History, 1935

Visitors from the Deep Past

For untold generations, before the invention of written history, people lived in small bands of relatives numbering, at most, a few dozen members. Our distant ancestors were merely creatures among other creatures, struggling to survive in an untamed wilderness. Called “hunter-gatherers” by modern social scientists, they were nomads, wanderers, people without a fixed place to live or call home. Each band had little contact with other bands, going from place to place, hunting animals and gathering roots, nuts, berries, and fruits to eat.

Unable to preserve or store food, nomads had to move continually, and on foot, to find their next meal. Without the wheel, a later invention, they also lacked draft animals; they kept no animals, except dogs, used for hunting and, in a pinch, for a meal. They carried their few possessions strapped to their backs or lashed between two wooden poles, which the women dragged along the ground. The able-bodied men walked ahead, armed with clubs, stone-tipped spears, and bows and arrows, eyes peeled for danger or for game to pursue. Camps usually were just overnight stops to eat and sleep. But if the hunting in an area was good, the band might stay for a few days longer to butcher a kill, fill their bellies, and rest up for the trek ahead.

Hunting accidents, wounds, falls from trees, feuds within a band, and clashes with other bands took a steady toll. Still, the nomadic lifestyle had one advantage: it limited the impact of infectious diseases carried by animals.

All types of living beings have diseases that afflict them alone. Sometimes, however, a disease attacking one life-form “crosses over” and infects another life-form. Ancient nomads did not live amid heaps of rubbish, their own waste, and polluted water. After a few days at a campsite, they moved on, leaving behind any disease-causing microbes that might be around. If a disease crossed over to, say, a hunter, he might die. The disease might even infect the entire band, killing everyone. But that would be the end of the disease; it stopped when there was no one left to infect. It could flourish only by becoming a “crowd disease,” infecting a population large enough to allow victims to pass it to the healthy.1

About 11,000 years ago, humankind reached a critical turning point. Across the world, big game animals—mastodons, giant sloths, and saber-toothed tigers—became extinct, probably due to over-killing by the hunters themselves. Naturally, as food became scarcer, nomads sought other ways of feeding themselves. Many began to experiment, growing wild plants like wheat, barley, and rice for food. They also learned to domesticate wild animals; that is, to tame and raise them. Cattle, horses, oxen, sheep, goats, pigs, ducks, geese, and chickens: all became important food sources, and some, like horses and oxen, became work animals.

This turning point, called the Agricultural Revolution, placed new demands on people. Above all, it required the cooperation of several groups living close together. Of necessity, hunter-gatherer bands settled into permanent communities when they...