Free The Chimp and the River: How AIDS Emerged from an African Forest by David Quammen

Cameroon, and last again, Beatrice H. Hahn. The data were fascinating, the conclusions were judicious, the language was careful and tight. Near the end, though, the authors let supposition fly:
    We show here that the SIV cpz Ptt strain that gave rise to HIV-1 group M belonged to a viral lineage that persists today in P. t. troglodytes apes in southeastern Cameroon. That virus was probably transmitted locally. From there it appears to have made its way via the Sangha River (or other tributaries) south to the Congo River and on to Kinshasa where the group M pandemic was probably spawned.
    But the phrase “transmitted locally” was opaque. What mechanism, what circumstances? How did those crucial events occur and proceed?
    Hahn herself, along with three coauthors, had addressed that back in 2000, when she first argued the idea that AIDS is a zoonosis: “In humans, direct exposure to animal blood and secretions as a result of hunting, butchering, or other activities (such as consumption of uncooked contaminated meat) provides a plausible explanation for the transmission.” She was alluding to the cut-hunter hypothesis. More recently she addressed it again: “The likeliest route of chimpanzee-to-human transmission would have been through exposure to infected blood and body fluids during the butchery of bushmeat.” A man kills a chimpanzee and dresses it out, hacks it up, in the course of which he suffers blood-to-blood contact through a cut on his hand. SIV cpz passesacross the species boundary, from chimp to human, and taking hold in the new host becomes HIV. This event is unknowable in its particulars but it’s plausible, and it fits the established facts. Some variant of the cut-hunter scenario, occurring in a forest of southeastern Cameroon around 1908, would account not just for Keele’s data but also for Michael Worobey’s timeline. But then what? One man in southeastern Cameroon is infected.
    “If the spillover occurred there,” I asked Hahn, “how was it that the epidemic began in Kinshasa?”
    “Well, there are lots of rivers going down from that region to Kinshasa,” she said. “And the speculation, the hypothesis, is that is how the virus traveled—in people, not in apes. It wasn’t the apes that got into the canoe for a little visit of Kinshasa. It was the people who carried the virus down, most likely.” Sure, she acknowledged, there was a slim chance that someone might have brought a live chimp, captive, infected, all the way down from the Cameroonian wedge—“but I think it is highly unlikely.” More likely the virus traveled in humans.
    Sexual contacts in the villages kept the chain of infection alive, though barely, by this line of speculation, and the disease didn’t explode as a notable outbreak—not for a long while. When someone died of immunodeficiency, the death may have seemed unremarkable amid all other sources of mortality. Life was hard, life was perilous, life expectancy was short even apart from the new disease, and many of those earliest HIV-positive people may have succumbed to other causes before their immune systems failed. There was no epidemic. But the chain of infection sustained itself. Each HIV-positive person infected, on average, at least one other person. The virus seems to have traveled just as people traveled in those days: mainly by river. It made its way out of southeastern Cameroon along the headwaters of the Sangha River, then down the Sangha to the Congo, then down the Congo to Brazzaville and Léopoldville,the two colonial towns on either side of a huge broadening of the river, which was then known as the Stanley Pool. “Once it got into an urban population,” Hahn said, “it had an opportunity to spread.”
    But still it moved slowly, like a locomotive just leaving the station. Léopoldville contained fewer than ten thousand people in 1908, and Brazzaville was even smaller. Sexual mores and the fluidity of interactions were unlike what prevailed in the