protist or plant, all embedded within ecological relationships that limit their abundance and their geographical range. Ebola and Marburg and Lassa and monkeypox and the precursors of the human immunodeficiency viruses represent just a minuscule sample of what’s there, of the myriad other viruses as yet undiscovered, within hosts that in many cases are as yet undiscovered themselves. Viruses can only replicate inside the living cells of some other organism. Commonly they inhabit one kind of animal or plant, with whom their relations are intimate, ancient, and often (but not always) commensal. That is to say, dependent but benign. They don’t live independently. They don’t cause commotion. They might kill some monkeys or birds once in a while, but those carcasses are quickly absorbed by the forest. We humans seldom have occasion to notice.
Three: But now the disruption of natural ecosystems seems more and more to be unloosing such microbes into a wider world. When the trees fall and the native animals are slaughtered, the native germs fly like dust from a demolished warehouse. A parasitic microbe, thus jostled, evicted, deprived of its habitual host, has two options—to find a new host, a new kind of host . . . or to go extinct. It’s not that they target us especially. It’s that we are so obtrusively, abundantly available. “If you look at the world from the point of view of a hungry virus,” the historian William H. McNeill has noted, “or even a bacterium—we offer a magnificent feeding ground with all our billions of human bodies, where, in the very recent past, there were only half as many people. In some 25 or 27 years, we have doubled in number. A marvelous target for any organism that can adapt itself to invading us.” Viruses, especially those of a certain sort—those whose genomes consist of RNA rather than DNA, leaving them more prone to mutation—are highly and rapidly adaptive.
All these factors have yielded not just novel infections and dramatic little outbreaks but also new epidemics and pandemics, of which the most gruesome, catastrophic, and infamous is the one caused by a lineage of virus known to scientists as HIV-1 group M. That’s the lineage of HIV (among twelve different sorts) that accounts for most of the worldwide AIDS pandemic. It has already killed 30 million humans since the disease was noticed three decades ago; roughly 34 million other humans are presently infected. Despite the breadth of its impact, most people are unaware of the fateful combination of circumstances that brought HIV-1 group M out of one remote region of African forest, where its precursor lurked as a seemingly harmless infection of chimpanzees, into human history. Most people don’t know that the 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