The HIV-1 virus that went on to infect 75 million people, jumped from a chimpanzee to a human in the 1920s. This happened in South Eastern Cameroon and it then travelled to Kinshasa (then Léopoldville), from where a subtype (C) was transported to the mining town Lubumbashi (then Élisabethville) in 1937, and from there to other places in Africa. The CLAHRC WM director likely encountered cases while he was a medical student in Johannesburg. At that time AIDS was unknown. However, Congolese men who had come to work in South Africa’s gold mines presented at Baragwanath Hospital with a disease characterised by wasting opportunistic infections, and Kaposi’s sarcoma. It was given the name “slim” (meaning thin in Afrikaans) disease. Sub-type B meanwhile, made its appearance in Kinshasa around 1944 and jumped to the United States in the 1960s, and hence to the rest of the world, including Africa where the sub-types B and C now co-exist.
How do we know all this? The emergence of HIV can be traced using archive human material to create family trees (phylogenies) and place dates on the origin and branches using a molecular clock (based on the mutation rate of viral DNA). 
— Richard Lilford, CLAHRC WM Director
- Cohen J. Early AIDS virus may have ridden Africa’s rails. Science. 2014; 346(6205): 21-2.
- Faria NR, Rambaut A, Suchard MA. The early spread and epidemic ignition of HIV-1 in human populations. Science. 2014; 346(6205): 56-61.