THESE WORDS, WRITTEN by a man from the Italian city of Siena, are one of many heartrending descriptions left by survivors of the epidemic of bubonic plague that scoured Europe, North Africa, India, and Asia in the mid–fourteenth century. That epidemic killed about a third of the people in Europe in less than ten years. It was later called the Black Death, but at the time, Europeans knew it simply as the Great Dying. It was one of many epidemics given similar terrifying names in different times and places.

An ancient fear returns

Until the twentieth century, infectious diseases—diseases caused by living things, usually microscopic, that invade the body—killed more human beings than war or famine. Often these diseases were simply an everyday hazard of life, slaying a few people here and there. Many infectious diseases, however, are also contagious (able to...