The Black Death, smallpox, and AIDS have been global in their scope. Bubonic plague began somewhere in central Asia and came west with its carriers—fleas that lived on rats. The disease was first reported in the southern Ukraine on the Black Sea in 1347. In October 1347 an Italian merchant ship arrived in Messina, Sicily, with its crew and passengers either dead or dying from the disease. It spread to the Italian mainland—first in its ports and then inland to the major cities. It entered France through Marseilles and other ports and spread rapidly to Avignon and Paris. By the time the pandemic ended in 1349 most of Europe had been infected. Outbreaks would recur at intervals during the centuries to follow: in London in 1664, for example, and in China in the late nineteenth century.

Smallpox became endemic in ancient Egypt and may have been brought back...