Before people knew about germs, the chances of catching a disease and dying from it were much greater than they are today. No one realized that millions of germs swarmed everywhere-in the air people breathed, the water they drank, the food they ate, the soil they tilled, and even on their own bodies. Dangerous germs passed freely from person to person, from house to house, and from village to village. As a result, periodic outbreaks of crippling or deadly diseases have occurred throughout history. Epidemics of measles, yellow fever, bubonic plague, leprosy, cholera, typhus, smallpox, and other debilitating maladies regularly wiped out thousands or millions of people at a time.

Occasionally a literate person jotted down an eyewitness account of the devastation wrought by such an epidemic, and some of these accounts have survived to the present. The Greek historian...