For thousands of years, smallpox was one of the world's most dreaded scourges. Caused by the Variola virus, this highly contagious disease is characterized by an extensive burning rash that erupts into pus-filled pocks, or pustules, that smell like rotting flesh. In 1634, the Puritan leader William Bradford wrote of Native Americans suffering from smallpox: "They lie on hard mats, the pox breaking and mattering [oozing pus], and running one into another, their skin cleaving [sticking] . . . to the mats they lie on; when they turn them, a whole side will flea [peel] off at once . . . and they will be all of a gore blood, most fearful to behold."Many smallpox victims died horribly, while survivors were scarred, and sometimes blinded, for life. An early account from Brazil described "pox that were so rotten and poisonous that the...