Smallpox is a severe, contagious viral disease. The scientific name for the virus that causes smallpox, Variola, comes from the Latin word varius, which means "spotted." Variola has no animal carrier; it infects only humans. It is transmitted from person to person through the air. Variola spreads when smallpox sufferers talk, cough, or sneeze, or when they leak pus from ruptured pocks onto clothes, bedding, and other items. Such contaminated objects can remain infectious for months and carry Variola over long distances.

In the 1880s, for example, a smallpox outbreak in Pecos, Texas, was traced to a silk handkerchief lying along the railroad track, which was thought to have been thrown from a passenger train. Victims' corpses can also spread Variola to people who prepare the corpses for burial.

Variola viruses enter people when they inhale, coming in through the nose and...