The strain of H.I.V. responsible for almost all AIDS cases in the United States, which was carried from Zaire to Haiti around 1967, spread from there to New York City around 1971

From New York, it spread to San Francisco around 1976. H.I.V. was carried from Kinshasa to Haiti in the 1960s — most likely by one of the thousands of Haitian civil servants recruited by the United Nations to work in the former Belgian Congo after colonial rule collapsed. Africa has a dozen H.I.V. groups, and Haiti’s epidemic came from one of those.