Excerpt from the new book Spillover, on understanding the threat of RNA viruses like Marburg, Ebola, West Nile and SARS—and how humans can help contain them:

“During the early 20th century, disease scientists from the Rockefeller Foundation and other institutions conceived the ambitious goal of eradicating some infectious diseases entirely. They tried hard with yellow fever, spending millions of dollars and many years of effort, and failed. They tried with malaria and failed. They tried later with smallpox and succeeded. Why? The differences among those three diseases are many and complex, but probably the most crucial one is that smallpox resided neither in a reservoir host nor in a vector, such as a mosquito or tick. Its ecology was simple. It existed in humans—in humans only—and was therefore much easier to eradicate. The campaign to eradicate polio, begun in 1998 by WHO and other institutions, is a realistic effort for the same reason: Polio isn’t zoonotic. Eradicating a zoonotic disease, whether a directly transmitted one like Ebola or an insect-vectored one such as yellow fever, is much more complicated. Do you exterminate the pathogen by exterminating the species of bat or primate or mosquito in which it resides? Not easily, you don’t, and not without raising an outcry. The notion of eradicating chimpanzees as a step toward preventing the future spillover of another HIV would provoke a deep and bitter discussion, to put it mildly.

“That’s the salubrious thing about zoonotic diseases: They remind us, as St. Francis did, that we humans are inseparable from the natural world. In fact, there is no ‘natural world,’ it’s a bad and artificial phrase.”