SIV is the monkey form of HIV and stands for Simian immunodeficiency virus. It is suspected that all monkeys in Africa are infected with SIV. Monkeys contain a gene which prevents SIV from becoming AIDS.
Six families of SIV are known; SIVsm, SIVagm, SIVmnd, SIVcpz, SIVsyk, and SIVrcm.
6 families of monkeys in sub-saharan Africa, none of which have had contact with the others for thousands of years, have all developed forms of SIV. This indicates that SIV is an ancient problem in Africa that existed long before the disease erupted into the human population in 1981.
It is generally accepted within the scientific community that HIV -1 and HIV -2 descended from strands of SIV. SIV and HIV are both part of a family of viruses known as lentiviruses. HIV-2 is almost identical to SIVsm, a strand of SIV found in the sooty mangabey monkey or the green monkey which can be found in western Africa.The stronger strain of HIV, HIV-1, is almost identical to SIVcpz. SIVcpz is found in a species of chimpanzees known as Pan troglodytes troglodytes which can be found Central-Western Africa. Some researchers claim that this species of chimpanzee was responsible for HIV-1 and at some point SIVcpz jumped the species barrier. However, this theory is often questioned as SIVcpz is a rarer strand that rarely infect chimpanzees
The other main theory of how SIV jumped the species barrier was published in 2003 by Professor Paul Sharp of Nottingham University and Beatrice Hahn of the University of Alabama. Their research indicated that a red-capped mangabey and a greater spot nosed monkey, both infected with their forms of SIV, had sex and the two strands of the virus combined to create a 3rd type of hybrid virus that was capable of infecting humans within them both. Chimpanzees became infected with this disease when they hunted and killed the smaller monkeys. When humans killed the chimpanzees and ate them, they became infected with the virus.
The process of diseases jumping the species barrier is known as zoonosis.