Spillover: Animal Infections and the Next Human Pandemic by David Quammen Read Free Book Online

real, full story of AIDS doesn’t begin among American homosexuals in 1981, or in a few big African cities during the early 1960s, but at the headwaters of a jungle river called the Sangha, in southeastern Cameroon, half a century earlier. Even fewer people have caught wind of the startling discoveries that, just within the past several years, have added detail and transformative insight to that story. Those discoveries will get their place later (“The Chimp and the River”) in this account. For now I’ll just note that, even if the subject of zoonotic spillover addressed nothing but the happenstance of AIDS, it would obviously command serious attention. But as mentioned already, the subject addresses much more—other pandemics and catastrophic diseases of the past (plague, influenza), of the present (malaria, influenza), and of the future.
    Diseases of the future, needless to say, are a matter of high concern to public health officials and scientists. There’s no reason to assume that AIDS will stand unique, in our time, as the only such global disaster caused by a strange microbe emerging from some other animal. Some knowledgeable and gloomy prognosticators even speak of the Next Big One as an inevitability. (If you’re a seismologist in California, the Next Big One is an earthquake that drops San Francisco into the sea, but in this realm of discourse it’s a vastly lethal pandemic.) Will the Next Big One be caused by a virus? Will the Next Big One come out of a rainforest or a market in southern China? Will the Next Big One kill 30 or 40 million people? The concept by now is so codified, in fact, that we could think of it as the NBO. The chief difference between HIV-1 and the NBO may turn out to be that HIV-1 does its killing so slowly. Most other new viruses work fast.
    I’ve been using the words “emergence” and “emerging” as though they are everyday language, and maybe they are. Among the experts, they’re certainly common parlance. There’s even a journal dedicated to the subject, Emerging Infectious Diseases ,published monthly by the CDC. But a precise definition of “emergence” might be useful here. Several have been offered in the scientific literature. The one I prefer simply says that an emerging disease is “an infectious disease whose incidence is increasing following its first introduction into a new host population.” The key words, of course, are “infectious,” “increasing,” and “new host.” A re-emerging disease is one “whose incidence is increasing in an existing host population as a result of long-term changes in its underlying epidemiology.” Tuberculosis is re-emerging as a severe problem, especially in Africa, as the TB bacterium exploits a new opportunity: infecting AIDS patients whose immune systems are disabled. Yellow fever re-emerges among humans wherever Aedes aegypti mosquitoes are allowed to resume carrying the virus between infected monkeys and uninfected people. Dengue, also dependent on mosquito bites for transmission and native monkeys as reservoirs, re-emerged in Southeast Asia after World War II due at least partly to increased urbanization, wider travel, lax wastewater management, inefficient mosquito control, and other factors.
    Emergence and spillover are distinct concepts but interconnected. “Spillover” is the term used by disease ecologists (it has a different use for economists) to denote the moment when a pathogen passes from members of one species, as host, into members of another. It’s a focused event. Hendra virus spilled over into Drama Series (from bats) and then into Vic Rail (from horses) in September 1994. Emergence is a process, a trend. AIDS emerged during the late twentieth century. (Or was it the early twentieth century? I’ll return to that question.) Spillover leads to emergence when an alien bug, having infected some members of a new host species, thrives in that species and spreads among it. In this sense, the strict sense,