I hope some people with medical knowledge get in on this thread. It seems logical that any disease would be around for awhile before it reached a critical point where medical folks would be looking for a new explanation for unaccountable diseases.
My recollection is that after the AIDS epidemic exploded, researches looked back and found there were some very early cases. It just didn't take off until it found a vulnerable host population.
Yes, I believe you are right about looking back and finding earlier AIDS patients. It went back much earlier, IIRC.