Health

THE global pandemic could have been so different. After AIDS was recognised in the US in 1981, many cases were found in Haitians, but it was never clear how Haiti figured in the epidemic.

Now Thomas Gilbert at the University of Arizona at Tucson and colleagues have analysed HIV samples collected from five Haitians living in Miami in 1982, and their relationship to 117 HIV samples from 19 other countries. The resulting "family tree" makes it clear that a virus originating in Haiti was the ancestor of virtually all HIV outside Africa, making Haiti the "stepping stone" from Africa to the world (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0705329104).

Comparison with African viruses showed HIV arrived in Haiti, probably in one person, in about 1966, at a time when many Haitians were working in newly independent Congo. Then in 1969 HIV spread to the US - again in one person - where it circulated, unrecognised, for 12 years. The team thinks it may have remained invisible until it reached gay communities - compact groups that spread the virus efficiently - which led to visible clusters of the disease. The virus's descendants caused the "vast majority" of infections outside Africa, they say.

The analyses reveal several "dead ends"; HIV strains that left Haiti but never got established. The difference between those viruses and the wildly successful pandemic strain seems to have been luck - there don't appear to be any genetic differences that made the pandemic strain fitter.

HIV and AIDS - Learn more about the worst pandemic in human history in our continuously updated special report.

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