Here's a tidbit from avert.org :
The 'Hunter' Theory
The most commonly accepted theory is that of the 'hunter'. In this scenario, SIVcpz was transferred to humans as a result of chimps being killed and eaten or their blood getting into cuts or wounds on the hunter. Normally the hunter's body would have fought off SIV, but on a few occasions it adapted itself within its new human host and become HIV-1. The fact that there were several different early strains of HIV, each with a slightly different genetic make-up (the most common of which was HIV-1 group M), would support this theory: every time it passed from a chimpanzee to a man, it would have developed in a slightly different way within his body, and thus produced a slightly different strain.
An article published in The Lancet in 20043, also shows how retroviral transfer from primates to hunters is still occurring even today. In a sample of 1099 individuals in Cameroon , they discovered to ten (1%) were infected with SFV (Simian Foamy Virus), an illness which, like SIV, was previously thought only to infect primates. All these infections were believed to have been acquired through the butchering and consumption of monkey and ape meat. Discoveries such as this have led to calls for an outright ban on bushmeat hunting to prevent simian viruses being passed to humans.
The Oral Polio Vaccine (OPV) theory
Could production of the oral polio vaccine
have contributed to the spread of HIV?
Some other rather controversial theories have contended that HIV was transferred iatrogenically (i.e. via medical experiments). One particularly well-publicised idea is that polio vaccines played a role in the transfer.
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.
However, in February 2000 the Wistar Institute in Philadelphia (one of the original places that developed the Chat vaccine) announced that it had discovered in its stores a phial of polio vaccine that had been used as part of the program. The vaccine was subsequently analysed and in April 2001 it was announced4 that no trace had been found of either HIV or chimpanzee SIV. A second analysis5 confirmed that only macaque monkey kidney cells, which cannot be infected with SIV or HIV, were used to make Chat. While this is just one phial of many, most have taken its existence to mean that the OPV vaccine theory is not possible.
The fact that the OPV theory accounts for just one (group M) of several different groups of HIV also suggests that transferral must have happened in other ways too.
The final element that suggests that the OPV theory is not credible as the sole method of transmission is the argument that HIV existed in humans before the vaccine trials were ever carried out. More about when HIV came into being can be found below.
The Contaminated Needle Theory
This is an extension of the original 'hunter' theory. In the 1950s, the use of disposable plastic syringes became commonplace around the world as a cheap, sterile way to administer medicines. However, to African healthcare professionals working on inoculation and other medical programmes, the huge quantities of syringes needed would have been very costly. It is therefore likely that one single syringe would have been used to inject multiple patients without any sterilisation in between. This would rapidly have transferred any viral particles (within a hunter's blood for example) from one person to another, creating huge potential for the virus to mutate and replicate in each new individual it entered, even if the SIV within the original person infected had not yet converted to HIV.
The Colonialism Theory
The colonialism or 'Heart of Darkness' theory, is one of the more recent theories to have entered into the debate. It is again based on the basic 'hunter' premise, but more thoroughly explains how this original infection could have led to an epidemic. It was first proposed in 2000 by Jim Moore, an American specialist in primate behaviour, who published his findings in the journal AIDS Research and Human Retroviruses.6
During the late 19th and early 20th century, much of Africa was ruled by colonial forces. In areas such as French Equatorial Africa and the Belgian Congo, colonial rule was particularly harsh and many Africans were forced into labour camps where sanitation was poor, food was scare and physical demands were extreme. These factors alone would have been sufficient to create poor health in anyone, so SIV could easily have infiltrated the labour force and taken advantage of their weakened immune systems to become HIV. A stray and perhaps sick chimpanzee with SIV would have made a welcome extra source of food for the workers.
Moore also believes that many of the labourers would have been inoculated with unsterile needles against diseases such as smallpox (to keep them alive and working), and that many of the camps actively employed prostitutes to keep the workers happy, creating numerous possibilities for onward transmission. A large number of labourers would have died before they even developed the first symptoms of AIDS, and those that did get sick would not have stood out as any different in an already disease-ridden population. Even if they had been identified, all evidence (including medical records) that the camps existed was destroyed to cover up the fact that a staggering 50% of the local population were wiped out there.
One final factor Moore uses to support his theory, is the fact that the labour camps were set up around the time that HIV was first believed to have passed into humans - the early part of the 20th century.
The Conspiracy Theory
Some say that HIV is a 'conspiracy theory' or that it is 'man-made'. A recent survey carried out in the US for example, identified a significant number of African Americans who believe HIV was manufactured as part of a biological warfare programme, designed to wipe out large numbers of black and homosexual people.7 Many say this was done under the auspices of the US federal 'Special Cancer Virus Program' (SCVP), possibly with the help of the CIA. Some even believe that the virus was spread (either deliberately or inadvertently) to thousands of people all over the world through the smallpox inoculation programme, or to gay men through Hepatitis B vaccine trials. While none of these theories can be definitively disproved, the evidence they are based on is tenuous at best, and often ignores the clear link between SIV and HIV, or the fact that the virus has been identified in people as far back as 1959. They also fail to take into consideration the lack of genetic-engineering technology available to 'create' the virus at the time that AIDS first appeared.