Where did HIV come from, and why did it take off only in the 20th century?

The pandemic form of HIV came from apes. The closest relatives of HIV-1 are simian immunodeficiency viruses (SIVs) infecting wild chimpanzees and gorillas in West Central Africa, and the pandemic strain, group M, derives from a chimpanzee virus in Cameroon. In the bushmeat theory, the virus passed to a hunter who was cut or injured while hunting or butchering an infected animal, and the present pandemic had its origins in group M in Léopoldville in the Belgian Congo, now Kinshasa, in the 1920s.

The harder question is why then. Bushmeat practices were widespread, and human SIV infections were common, yet only four HIV groups spread considerably. One proposal, from Beatrice Hahn, Paul Sharp and colleagues, points to changes in population and behaviour under colonial rule: after the Scramble for Africa began in the 1880s, colonial powers established cities and stations and hastily recruited a largely male labour force for ports, railways and plantations. Another, from Preston Marx and colleagues, is that unsafe or unsterile injections passed the virus rapidly from person to person.

One early clue is a preserved blood sample taken in 1959 from a man in Léopoldville, one of the earliest documented HIV-1 infections.

Its arrival in North America is surrounded by a myth. Gaëtan Dugas, a Canadian airline steward, was labelled 'Patient O', for 'outside Southern California', in an early CDC study, and a 1987 book turned that into 'Patient Zero'. Many people came to see him as the man who brought HIV to North America. He was not: HIV reached New York City around 1971, and he did not start work at Air Canada until 1974.