So just how was the virus spread to humans? There is some suggestion that poachers may have killed the infected chimps and then eaten them or even had sex with them.
Another extremely controversial theory is that HIV was spread via the polio vaccine. In his explosive book The River - A Journey to the Source of AIDS and HIV British journalist Edward Hooper claims that a polio vaccine developed by Polish scientist Hilary Koprowski and given to people in Western Africa in the late 50s, is the root cause of the pandemic now gripping the African continent. He says that the vaccine was cultured from chimps infected with SIV.
This theory has been rubbished by some scientists who doubt whether HIV could be transmitted via a vaccine which is taken orally. There is no information to determine whether those who received the vaccines in the Congo went on to develop AIDS. And Hooper detractors claim the vaccine cultures came from monkeys and not chimps. The alleged infected vaccine batch has also been tested by scientists and found to contain no evidence of HIV, SIV or even chimp DNA. But Hooper is sticking to his guns. He claims he has evidence that chimps were shipped over to America from the Congo for development of the vaccine.
And he believes the vaccine tested by scientists to dispel his theory may not have been the original batch in question. At the end of the day, it's all speculation. Nobody really knows how and why the virus entered our society. Doctors have known about it since 1981 when people with symptoms first started emerging but it's likely that it was around for at least 20 years before that.
The great tragedy is that this failure to determine the root cause of HIV is a seemingly insurmountable obstacle to finding either a cure or a vaccine.