FROM THE BEGINNING of human development, history—that is, people's behavior, both as individuals and in groups—has shaped epidemics. Epidemics, in turn, have shaped history. The outcome of wars, the rise and fall of empires, have often depended more on microbes than on rulers or armies. Microbiologist Hans Zinsser has written that epidemic disease "has decided more [military] campaigns than Caesar, Hannibal, Napoleon" and all the other famous war leaders of history.

The birth of epidemics

Scientists who have studied the remains of early humans believe that these ancestors had infectious and contagious diseases. They seldom had epidemics, however, because they lived in small groups (a hundred people at most) and did not stay long in one place. Microbes therefore had little chance to infect, or invade the bodies of, large numbers of people at once.

Humans began encountering...