It is thought that it was transmitted from bites from the 'Green Monkey' in Africa. However, this has never been firmly established but Africa does appear to have been where the virus established itself. Tissue samples from the death of unknown origin (at the time) in the early 1960's of a Seaman having returned from Africa were re-tested in the 1980's and found to contain a virus of similar structure to the AIDS Virus!
One quite compelling theory is that it was a simian immunodepressive virus that mutated into a virulent human immunodepressive virus when trials of polio vaccines, which were made in monkey tissues, were trialled in Africa - in the areas where Aids was first noted.
Not sure how credible this reference is (it's from the University of Wollongong in NWS, Australia) but here's a very brief summary of how the thinking goes click here click here for the rather weighty book that discusses the theory exhaustively. I've read it - it's fascinating stuff.
This thread is now locked and can not be replied to.