Over the past few decades, our optimism — or perhaps hubris — that infectious disease had been conquered has been overturned so completely that emerging infections are now a new discipline, complete with their own journal. Even in the past year, severe acute respiratory syndrome, an entirely new disease of humans, has emerged, and monkeypox, a zoonosis, has appeared for the first time in the Americas. In 1999, the first cases of West Nile virus disease were recognized in New York, and the epidemic has since expanded, infecting about 400,000 Americans (and countless birds, mammals, and even some reptiles) in . . .