directly detected nor traced to wild carnivores until much later. During the early twentieth 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. 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 1988 by WHO and other institutions, is a realistic effort for the same reason: Polio isn’t zoonotic. And malaria is now targeted again. The Bill and Melinda Gates Foundation announced, in 2007, a new long-term initiative to eradicate that disease. It’s an admirable goal, a generously imaginative dream, but a person is left to wonder how Mr. and Mrs. Gates and their scientific advisers propose to deal with Plasmodium knowlesi. Do you exterminate the parasite by killing off its reservoir hosts, or do you somehow apply your therapeutics to those hosts, curing every macaque in the forests of Borneo?
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. There is only the world. Humankind is part of that world, as are the ebolaviruses, as are the influenzas and the HIVs, as are Nipah and Hendra and SARS, as are chimpanzees and bats and palm civets and bar-headed geese, as is the next murderous virus—the one we haven’t yet detected.
I don’t say these things about the ineradicability of zoonoses to render you hopeless and depressed. Nor am I trying to be scary for the sake of scariness. The purpose of this book is not to make you more worried. The purpose of this book is to make you more smart. That’s what most distinguishes humans from, say, tent caterpillars and gypsy moths. Unlike them, we can be pretty smart.
Greg Dwyer came around to this point during our talk in Chicago. He had studied all the famous mathematical models proposed to explain disease outbreaks in humans—Anderson and May, Kermack and McKendrick, George MacDonald, John Brownlee, and the others. He had noted the crucial effect of individual behavior on rate of transmission. He had recognized that what people do as individuals, what moths do as individuals, has a large effect on R0. The transmission of HIV, for instance, Dwyer said, “depends on human behavior.” Who could argue? It has been proven. Consult the changes in rate of transmission among American gay men, among the general populace of Uganda, or among sex workers in Thailand. The transmission of SARS, Dwyer said, seems to depend much on superspreaders—and their behavior, not to mention the behavior of people around them, can be various. The mathematical ecologist’s term for variousness of behavior is “heterogeneity,” and Dwyer’s models have shown that heterogeneity of behavior, even among forest insects, let alone among humans, can be very important in damping the spread of infectious disease.
“If you hold mean transmission rate constant,” he told me, “just adding heterogeneity by itself will tend to reduce the overall infection rate.” That sounds dry. What it means is that individual effort, individual discernment, individual choice can have huge effects in averting the catastrophes that might otherwise sweep through a herd. An individual gypsy moth may inherit a slightly superior ability to avoid smears of NPV as it grazes on a leaf. An individual human may choose not to drink the palm sap, not to eat the chimpanzee, not to pen the pig beneath mango trees, not to clear the horse’s windpipe with his bare hand, not to have unprotected sex with the prostitute, not to share the needle in a shooting gallery, not to cough without covering her mouth, not to board a plane while feeling ill, or not to coop his chickens along with his ducks. “Any tiny little thing that people do,” Dwyer said, if it makes them different from one another, from the idealized standard of herd behavior, “is going to reduce infection rates.” This was after I had asked him to consider The Analogy and he had pushed his brain against it for half an hour.