Because of the constant human encroachment on natural habitat and the fact that we travel so widely and import so much from so many places, once isolated diseases are now able to spread easily among large human populations.
Many African tribesmen eat "bushmeat"--i.e., they kill animals like monkeys, chimpanzees, and gorillas for meat. The virus could jump from monkey to man pretty easily while the hunter is skinning and cutting up the meat of an infected monkey or chimpanzee. But human infections of this sort remained isolated and thus unknown until the areas where they occurred were linked up with the modern world. Then, modern global transportation made it easy for the infection to spread to urban centers in Africa and from urban centers in Africa to urban centers all over the world, while the sexual revolution made its sexual transmission more rapid than it would have been in a more sexually repressed era.
I have read that in the distant past up to 90% of the species susceptible to the virus died of it, so that those who are descended from the survivors have at least some degree of immunity against its more lethal effects and live with it as a chronic though not necessarily fatal condition. In fact, in many simian species, infection doesn't cause any symptoms at all, while others suffer from varying degrees of illness, and some do develop full-blown simian AIDS and die of it the way humans do of the human variation of AIDS.
Here is a bit of information, but there is plenty more all over the web:
http://www.springerlink.com/content/6771m57034n06162/
HIV type 1 (HIV-1) and type 2 (HIV-2) are the result of several cross-species transmissions from primates to humans. Recently, the ancestral strains of HIV-1 groups M and N were shown to still persist in today’s wild chimpanzee populations (Pan troglodytes troglodytes) in south Cameroon. Lately, HIV-1 group O-related viruses have been identified in western gorillas (Gorilla gorilla), called SIVgor, but chimpanzees are most likely the original reservoir of this simian immunodeficiency virus (SIV) infection. HIV-2 is the result of at least eight distinct cross-species transmissions of SIV from sooty mangabeys (Cercocebus atys) in West Africa. Although the origin of HIV-1 and HIV-2 became clearer, some important questions concerning pathogenicity and epidemic spread of certain HIV/SIV variants need to be further elucidated. Because humans are still exposed to a plethora of primate lentiviruses through hunting and handling of primate bushmeat, the possibility of additional zoonotic transfers of primate lentiviruses from other primates must be considered.
I found a really good, detailed article for you:
http://perspectivesinmedicine.org/content/2/1/a007153.full
The government is often involved in dark and evil deeds, but we cannot blame the govrnment for creating HIV/AIDS!
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