Simian Immunodeficiency Virus (SIV) from chimpanzee most likely mutated in Human Immunodeficiency Virus (HIV) to cause AIDS.

It is now generally accepted that HIV is a descendant of a Simian Immunodeficiency Virus because certain strains of SIVs bear a very close resemblance to HIV. It is also known that certain viruses can pass between species. Some of the most common theories, which describes about how the viral transfer between animals and humans takes place and how SIV became HIV in humans are explained in detail.

The Hunter Theory: It is the most commonly accepted theory. It is said that the virus (SIV) was transferred to humans as a result of chimps being killed and eaten or their blood getting into cuts or wounds on the hunter. SIV on a few occasions adapted itself within its new human host and become HIV. 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.

The Oral Polio Vaccine Theory: In this it is said that the virus was transmitted via various medical experiments (iatrogenically) especially through the polio vaccines. The oral polio vaccine called Chat was given to millions of people in the Belgian Congo, Ruanda and Urundi in the late 1950s. Then it was cultivated on kidney cells taken from the chimps infected with SIV in order to reproduce the vaccine. This is the main source of contamination, which later affected large number of people with HIV. But it was rejected as it was proved that only macaque monkey kidney cells, which cannot be infected with SIV or HIV were used to make Chat. Another reason is that HIV existed in humans before the vaccine trials were carried out.

The Contaminated Needle Theory: African healthcare professionals were using one single syringe to inject multiple patients without any sterilization in between. This could have rapidly have transferred infection from one individual to another resulting in mutation from SIV to HIV.

The Colonialism Theory: The colonial rule in Africa was particularly harsh and the locals were forced into labor camps where sanitation was poor and food was scare. SIV could easily have infiltrated the labor force and taken advantage of their weakened immune systems. Laborers were being inoculated with unsterile needles against diseases such as smallpox to keep them alive and working. Also many of the camps actively employed prostitutes to keep the workers happy. All these factors may have led to the transmission and development of AIDS as a disease.

The Conspiracy Theory: According to a survey, which was carried among African Americans it was found that HIV was manufactured as part of a biological warfare programme, designed to wipe out large numbers of black and homosexual people. There is no evidence to disprove it, cannot be accepted as there were no genetic engineering techniques at that time of emergence of AIDS.