Maybe they don't have goats in that part of the world.
Probably not the way everyone is thinking. A chimpanzee can easily tear a man's arms off. Screwing them would probably prove difficult.
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.
Agree with Drew here. The most likely means of transferrance is through contact with blood from chimpanzees killed for bushmeat.