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small river draining the northeastern Bolivian lowlands. The first recorded case of the disease came and went, almost unnoticed, as a bad but nonfatal fever afflicting a local farmer. This was during the wet season of 1959. More such illnesses, and worse, occurred in the same region over the following three years. Symptoms included fever and chills, nausea and vomiting, body aches, nosebleeds, and bleeding gums. It became known as El Tifu Negro (the Black Typhus, for the color of vomit and stool), and by late 1961 had struck 245 people, with a case fatality rate of 40 percent. It continued killing until the virus was isolated, its reservoir identified, and its dynamics of transmission understood well enough to be interrupted by preventive measures. Mouse trapping helped enormously. Most of the scientific work was done under difficult field conditions by a patched-together team of Americans and Bolivians, including an intense young scientist named Karl Johnson, pungently candid with his opinions, deeply enthralled by the dangerous beauty of viruses, who caught the disease himself and nearly died of it. This was before the Centers for Disease Control and Prevention (CDC) in Atlanta sent out well-equipped squads; Johnson and his colleagues invented their methods and tools as they went. Having struggled through his fever at a hospital in Panama, Karl Johnson would play a large and influential role in the longer saga of emerging pathogens.
    If you assembled a short list of the highlights and high anxieties of that saga within recent decades, it could include not just Machupo but also Marburg (1967), Lassa (1969), Ebola (1976, with Karl Johnson again prominently involved), HIV-1 (inferred in 1981, first isolated in 1983), HIV-2 (1986), Sin Nombre (1993), Hendra (1994), avian flu (1997), Nipah (1998), West Nile (1999), SARS (2003), and the much feared but anticlimactic swine flu of 2009. That’s a drama series more glutted and seething with virus than even Vic Rail’s poor mare.
    A person might construe this list as a sequence of dire but unrelated events—independent misfortunes that have happened to us, to humans, for one unfathomable reason and another. Seen that way, Machupo and the HIVs and SARS and the others are “acts of God” in the figurative (or literal) sense, grievous mishaps of a kind with earthquakes and volcanic eruptions and meteor impacts, which can be lamented and ameliorated but not avoided. That’s a passive, almost stoical way of viewing them. It’s also the wrong way.
    Make no mistake, they are connected, these disease outbreaks coming one after another. And they are not simply happening to us; they represent the unintended results of things we are doing . They reflect the convergence of two forms of crisis on our planet. The first crisis is ecological, the second is medical. As the two intersect, their joint consequences appear as a pattern of weird and terrible new diseases, emerging from unexpected sources and raising deep concern, deep foreboding, among the scientists who study them. How do such diseases leap from nonhuman animals into people, and why do they seem to be leaping more frequently in recent years? To put the matter in its starkest form: Human-caused ecological pressures and disruptions are bringing animal pathogens ever more into contact with human populations, while human technology and behavior are spreading those pathogens ever more widely and quickly. There are three elements to the situation.
    One: Mankind’s activities are causing the disintegration (a word chosen carefully) of natural ecosystems at a cataclysmic rate. We all know the rough outlines of that problem. By way of logging, road building, slash-and-burn agriculture, hunting and eating of wild animals (when Africans do that we call it “bushmeat” and impute a negative onus, though in America it’s merely “game”), clearing forest to create cattle pasture, mineral extraction, urban settlement, suburban sprawl,