humans or monkeys. She screened them using tests for both SIV and HIV. Despite one possible misstep involving a lab contamination, her team found what they had thought they might: a virus intermediate between HIV and SIV. With the code unblinded, Kanki learned that the positive results came from Senegalese prostitutes. In retrospect it made sense. Prostitutes are at high risk for any sexually transmitted virus, including a new one recently spilled into humans. And the density of the rural human population in Senegal, where African green monkeys are native, makes monkey-human interactions (crop-raiding by monkeys, hunting by humans) relatively frequent.
Furthermore, the new bug from Senegalese prostitutes wasn’t just halfway between HIV and SIV. It more closely resembled SIV strains from African green monkeys than it did the Montagnier-Gallo version of HIV. That was important but puzzling. Were there two distinct kinds of HIV?
Luc Montagnier now reenters the story. Having tussled with Gallo over the first HIV discovery, he converged more amicably with Essex and Kanki on this one. Using assay tools provided by the Harvard group, Montagnier and his colleagues screened the blood of a twenty-nine-year-old man from Guinea-Bissau, a tiny country, formerly a Portuguese colony, along the south border of Senegal. This man showed symptoms of AIDS (diarrhea, weight loss, swollen lymph nodes) but tested negative for HIV. He was hospitalized in Portugal, and his blood sample hand-delivered to Montagnier by a visiting Portuguese biologist. In Montagnier’s lab, the man’s serum again tested negative for antibodies to HIV. But from a culture of his white blood cells Montagnier’s group isolated a new human retrovirus, which looked very similar to what Essex and Kanki had found. In another patient, hospitalized in Paris but originally from Cape Verde, an island nation off the west coast of Senegal, the French team found more virus of the same type. Montagnier called the new thing LAV-2. Eventually, when all parties embraced the label HIV instead, it would be HIV-2. The original became HIV-1.
The paths of discovery may be sinuous, the labels may seem many, and maybe you can’t tell the players without a scorecard; but these details aren’t trivial. The difference between HIV-2 and HIV-1 is the difference between a nasty little West African disease and a global pandemic.
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During the late 1980s, as Kanki and Essex and other scientists studied HIV-2, a flurry of uncertainty arose about its provenance. Some challenged the idea that it was closely related to (and recently derived from) a retrovirus that infects African monkeys. An alternative view was that such a retrovirus had been present in the human lineage for as long as—or longer than—human time. Possibly it was already with us, a passenger riding the slow channels of evolution, when we diverged from our primate cousins. But that view left an unresolved conundrum: If the virus was an ancient parasite upon humans, unnoticed for millennia, how had it suddenly become so pathogenic?
Recent spillover seemed more likely. Still, the case against that idea got a boost in 1988, when a group of Japanese researchers sequenced the complete genome of SIV from an African green monkey. The animal came from Kenya. The nucleotide sequence of its retrovirus proved to be substantially different from the sequence for HIV-1, and different in roughly the same degree from HIV-2. So the monkey virus seemed no more closely related to the one human virus than to the other. That contradicted the notion that HIV-2 had lately emerged from an African green monkey. A commentary in the journal Nature, published to accompany the Japanese paper, celebrated this finding beneath a dogmatic headline: HUMAN AIDS VIRUS NOT FROM MONKEYS. But the headline was misleading to the point of falsity. Not from monkeys? Well, don’t be so sure. It turned out that researchers were just looking at the wrong kind of monkey.
Confusion came from two sources. For starters, the label “African green monkey” is a little vague. It encompasses a diversity of forms, sometimes also known as savannah monkeys, that occupy adjacent geographical ranges spread out across sub-Saharan Africa, from Senegal in the west to Ethiopia in the east and down into South Africa. At one time those forms were considered a “superspecies” under the name Cercopithecus aethiops. Nowadays, their differences having been more acutely gauged, they are classified into six distinct species within the genus Chlorocebus. The “African green monkey” sampled by the Japanese team, because it was “of Kenyan origin,” probably belonged to the species Chlorocebus pygerythrus. The species native to Senegal, on the