This article argues that HIV group O most likely started to spread (not originated due to, but spread) at a much greater rate in Cameroon in the mid-20th century as a result of "iatrogenic amplification".I can give my little humble contribution to this thread: I'm a trained NBC operator of Civil Protection [Nuclear Biological Chemical risk]. Regarding the main plagues and epidemics the operators receive a bit of historical background. HIV came from Camerum, along river Sangha in 1920, when a chimp transmitted the BA [biological agent] to a human being. Actually it wasn't HIV yet, for accuracy it was SIV [simian immunodeficiency virus ]. The source of the information we received was Oxford University and honestly I tend to trust such a source.
Now, it was a variant of that virus, the HIV-1 group M to be able to jump totally into the human environment.
A curiosity is that [and this is against conspiracy theories] in Camerun there was a different evolution called HIV group O which remained there. It's the difference about the diffusion of the "winner", group M, to be really interesting from
researchers, as for I know.
The Two-Phase Emergence of Non Pandemic HIV-1 Group O in Cameroon
It cites this earlier article from 2008, which is also interesting: