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

seems to have survived, but a South African nurse who looked after him sickened and died. Ebola virus had thereby emerged from Central Africa into the continent at large. The eventual tally from this third outbreak, encompassing Booué, Libreville, and Johannesburg, was sixty cases, of which forty-five were fatal. Rate of lethality? For that one, you can do the math in your head.
    Amid this welter of cases and details, a few common factors stand out: forest disruption at the site of the outbreak, dead apes as well as dead humans, secondary cases linked to hospital exposure or traditional healers, and a high case fatality rate, ranging from 60 to 75 percent. Sixty percent is extremely high for any infectious disease (except rabies); it’s probably higher, for instance, than mortalities from bubonic plague in medieval France at the worst moments of the Black Death.
    In the years since 1996, other outbreaks of Ebola virus disease have struck both people and gorillas within the region surrounding Mayibout 2. One area hit hard lies along the Mambili River, just over the Gabon border in northwestern Congo, another zone of dense forest encompassing several villages, a national park, and a recently created reserve known as the Lossi Gorilla Sanctuary. Mike Fay and I had walked through that area also, in March 2000, just four months before my rendezvous with him at the Minkébé inselbergs. In stark contrast to the emptiness of Minkébé, gorillas had been abundant within the Mambili drainage when we saw it. But two years later, in 2002, a team of researchers at Lossi began finding gorilla carcasses, some of which tested positive for antibodies to Ebola virus. (A positive test for antibodies is less compelling evidence than a find of live virus, but still suggestive.) Within a few months, 90 percent of the individual gorillas they had been studying (130 of 143 animals) had vanished. How many had simply run away? How many were dead? Extrapolating rather loosely from confirmed deaths and disappearances to overall toll throughout their study area, the researchers published a paper in Science under the forceful (but overconfident) headline: EBOLA OUTBREAK KILLED 5000 GORILLAS .
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    I n 2006 I returned to the Mambili River, this time with a team led by William B. (Billy) Karesh, then director of the Field Veterinary Program for the Wildlife Conservation Society (WCS) of New York and now filling a similar role at the EcoHealth Alliance. Billy Karesh is a veterinarian and an authority on zoonoses. He’s a peripatetic field man, raised in Charleston, South Carolina, nourished on Marlin Perkins, whose usual working uniform is a blue scrub shirt, a gimme cap, and a beard. An empiricist by disposition, he speaks quietly, barely moving his mouth, and avoids categorical pronouncements as though they might hurt his teeth. Often he wears a sly smile, suggesting amusement at the wonders of the world and the varied spectacle of human folly. But there was nothing amusing about his mission to the Mambili. He had come to shoot gorillas—not with bullets but with tranquilizer darts. He meant to draw blood samples and test them for antibodies to Ebola virus.
    Our destination was a site known as the Moba Bai complex, a group of natural clearings near the east bank of the upper Mambili, not far from the Lossi sanctuary. A bai in Francophone Africa is a marshy meadow, often featuring a salt lick, and surrounded by forest like a secret garden. In addition to Moba Bai, the namesake of this complex, there were three or four others nearby. Gorillas (and other wildlife) frequent such bais, which are waterlogged and sunny, because of the sodium-rich sedges and asters that grow beneath the open sky. We arrived at Moba, coming upstream on the Mambili, in an overloaded dugout pushed by a 40-horse outboard.
    The boat carried eleven of us and a formidable pile of gear. We had a gas-powered refrigerator, two liquid-nitrogen freezer tanks (for preserving samples),