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chemical pollution, nutrient runoff to the oceans, mining the oceans unsustainably for seafood, climate change, international marketing of the exported goods whose production requires any of the above, and other “civilizing” incursions upon natural landscape—by all such means, we are tearing ecosystems apart. This much isn’t new. Humans have been practicing most of those activities, using simple tools, for a very long time. But now, with 7 billion people alive and modern technology in their hands, the cumulative impacts are becoming critical. Tropical forests aren’t the only jeopardized ecosystems, but they’re the richest and most intricately structured. Within such ecosystems live millions of kinds of creatures, most of them unknown to science, unclassified into a species, or else barely identified and poorly understood.
    Two: Those millions of unknown creatures include viruses, bacteria, fungi, protists, and other organisms, many of which are parasitic. Students of virology now speak of the “virosphere,” a vast realm of organisms that probably dwarfs every other group. Many viruses, for instance, inhabit the forests of Central Africa, each parasitic upon a kind of bacterium or animal or fungus or 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