Despite its unsavory reputation as one of the legendary scourges of mankind, yellow fever is primarily a disease of animals-monkeys, in particular. In South America, the virus moves through the canopies of tropical rain forests in enormous waves. Carried by mosquitoes, its primary victims are howler monkeys, which are chimp-sized and notorious for being heard rather than seen. Researchers who study this jungle cycle of yellow fever say that they can tell when the waves of virus are rolling by because dead howlers start dropping out of trees. While the virus periodically finds its way into humans working in the rain forest, there hasn't been an urban epidemic in this hemisphere since 1942, a situation that has epidemiologists and public health experts holding their breath.

In their worst-case nightmares, a traveler or tourist contracts yellow fever in the Amazon, gets on a plane before symptoms appear and gets off 3,000 miles away, where he or she can be bitten by the local mosquitoes before the disease has been discovered or diagnosed, and the victim effectively quarantined. That such nightmares are not solely the stuff of imagination was demonstrated on the night of June 28, 1996, when a Swiss physician posted a case report on a Web site known as ProMED-Mail. The report was short and pithy: the story of a single unfortunate, unvaccinated tourist, who contracted yellow fever on a boat trip through the Amazon on April 5, and died 10 days later in a hospital in Basel. The fact that such episodes have not yet sparked a new wave of urban epidemics of yellow fever, according to New York State Health Department epidemiologist Jack Woodall, founder of ProMED-Mail, can be attributed to a single factor: "pure luck."