[jcranmer]

AIDS is caused by HIV, which is believed to have evolved from a virus infecting primates in Africa (probably more specific details than that exist, but I don't know them offhand).

Bubonic plague and polio both predate the existence of writing, and may well predate even agriculture.

[lovemenot]

HIV has at least one similarly plausible but unprovable origin story - concerning the unintended consequences of polio vaccination in the 1950s.

https://pubmed.ncbi.nlm.nih.gov/7935079/

[selimthegrim]

This has been disproven pretty thoroughly IIRC

[paiute]

https://en.m.wikipedia.org/wiki/Oral_polio_vaccine_AIDS_hypo...