If the plausible explanation for HIV?
Apes contract the predecessor, Simian Immunodeficiency Virus, through sex or contact with infected blood (for example, during fights), or by contact between bodily fluids and mucous membranes. HIV is a mutation of a lentivirus called SIVcpz. SIVcpz is a virus that infects chimpanzees, but it is itself a chimera of two different simian lentiviruses, suggesting it likely jumped species from other primates. Since chimpanzees have been observed hunting, killing, and eating monkeys, this is not too surprising. Coinfection of a chimpanzee with two different strains of lentivirus appears to have given rise to SIVcpz, which in turn mutated to HIV. SIVcpz doesnt kill chimpanzees. Infected cells undergo apoptosis very quickly after infection, so the virus never overwhelms the infected chimpanzees immune system. The mutated version, HIV, appears to interfere with apoptosis in human CD4 T-cells. In general, viruses have been with us as long as cellular life has existed. The best description Ive heard is they form a cloud around the tree of life. They were with us from the beginning, merrily coevolving alongside all life, and will be with us until the end.