Here's more:
From this source.
In his book, The River, the journalist Edward Hooper suggested that HIV could be traced to the testing of an oral polio vaccine called Chat, given to about a million people in the Belgian Congo , Ruanda and Urundi in the late 1950s. To be reproduced, live polio vaccine needs to be cultivated in living tissue, and Hooper's belief is that Chat was grown in kidney cells taken from local chimps infected with SIVcmz. This, he claims, would have resulted in the contamination of the vaccine with chimp SIV, and a large number of people subsequently becoming infected with HIV-1.
There's more about it on that site. Scientists discredited his work, but he could've been on to something.
**marking for reference....wondering what else is in vaccines we have no idea about
The vaccines in question were based on viruses cultivated in the kidney cells of primates known as green monkeys. That said, it is not certain whether SIV/HIV was present in that species as well as in chimpanzees....