"ONE OF THE great lies of the twentieth century," writes Canadian science historian Andrew Nikiforuk, is that "antibiotics, vaccines and doctors have saved us from pestilence [epidemic disease]." Far from being defeated, epidemics are making a powerful comeback today. Infectious disease, in fact, is the world's single most common cause of death. It is estimated to have accounted for 17 million of the 52 million deaths that occurred in 1995, for instance. Even in the United States, known for its medical technology, deaths from infectious diseases rose 58 percent between 1980 and 1992. By the late 1990s, infections were the third-largest killer of Americans, after heart disease and cancer.

Masses on the move

Many of the conditions that encourage modern epidemics are the same ones that spawned ancient plagues, but they now exist on an unprecedented scale. Perhaps the most important of these conditions is the...