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.

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