The following site is an excellent source for information on the Aids
virus. It is really worth reading.
http://hivinsite.ucsf.edu/InSite?page=pr-02-10&doc=2098.3ccesnip:
Where did HIV come from?
At this time, most evidence suggests that AIDS has its roots in Africa. In the early 1980s, Robert Gallo speculated that HIV crossed species to humans from an African green monkey. Although this idea has since been discredited, it is still widely believed that HIV came into the human population from one or more non-human primate species.
Scientists have long recognized the ability of certain viruses and other diseases to pass from animals to humans. This process is referred to as zoonosis. Once an animal disease has infected people, it may then be passed from human to human. The Ebola virus, (5) and the Marburg filovirus (6) are two diseases among many which were introduced into humans from animals through zoonotic transfer. Although it has not been proven that HIV came from primates, an SIV has been known to infect humans. (7)
Various means have been suggested through which HIV might have passed from other primates to humans. One theory is that the virus could have been transferred to humans while sooty mangabeys or chimpanzees were butchered for food or kept as pets. The most controversial theories contend that medical science played a role in introducing HIV into the human population. These hypotheses are so contentious because, if proven, the HIV epidemic would have begun as a result of human scientific error. Such a finding would be "incredibly damaging to the image of medicine and medical research," (8) to say the least.
The most well-known of the human error theories is that polio vaccines, which were given to many Africans in the 1950s, could have been contaminated with HIV. The vaccines were prepared using monkey kidneys. Although HIV has not been found in any of the vaccine batches which have been tested thus far, polio vaccines have been found to be contaminated with various other pathogens. One of these, Simian Virus 40 (SV40), infected millions of Americans, although it does not seem to be harmful to humans. (9) The World Health Organization has publicly refuted the arguments of scientists who suggest that the vaccines could have been the channel through which the virus crossed species. Despite this, the controversy continues. A recent letter in The Lancet hypothesized that polio vaccine stocks could have been contaminated by retroviruses. (10)