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Human immunodeficiency virus type 1 (HIV-1) is a lentivirus that kills immune system cells in humans, ultimately causing AIDS (acquired immune deficiency syndrome).  It is spread between hosts when blood or body fluid from a contaminated individual enters the bloodstream of another individual, e.g. by transfusion, sexual intercourse, breastfeeding, use of shared hypodermic needles.  Research suggests HIV-1 evolved from closely related primate viruses (simian immunodeficiency viruses, or SIVs).  A likely origin was viral recombination occurring in wild chimps infected simultaneously with more than one different SIV, and causing the formation of a new, hybrid SIV capable of being transmitted not just within chimpanzees, but also between chimpanzees, gorillas and humans.  This mutant hybrid crossed into humans not just once, but in four separate events, twice from chimpanzees (probably) and twice from gorillas (via a chimpanzee intermediate).  Each event created a genetically distinct HIV-1 group, termed group M, N, O, and P.  Of these, HIV-1 group M is the most common, responsible for virtually all HIV/AIDS cases worldwide.  Most recent genetic analysis indicates that HIV-1 M crossed to humans at the beginning of the 20th century from chimpanzee subspecies P. t. troglodytes in southeast Cameroon.  A likely vehicle of human infection was contact with contaminated chimp blood during butchery and consumption of bushmeat (Sharp and Hahn 2010; AVERT 2014).  

AIDS case reportings were rare until 1970 when the AIDS pandemic rapidly set in and by the 1980s AIDS had spread to five continents.  HIV-1 mutates rapidly.  Since its origin in humans a hundred years ago, multiple HIV-1 M strains with much genetic variability evolved and spread around the world, even before the first recognized HIV cases in the late 1950s. The genetic strains, which are generally concentrated in particular world regions, are classified into subtypes and hybrids of subtypes (called CRFs, Circulating Recombinant Forms).  Subtypes A and C are currently the most widespread.  Antiretroviral medications, which work by keeping HIV particles low so the immune system can function properly, have primarily been developed against subtype B.  These drugs appear equally effective on other HIV-1 subtypes, however research indicates that subtypes may evolve drug resistances differentially (Wainberg 2004; Martínez-Cajas et al. 2008).    

One other HIV type (HIV-2) exists, also causing AIDS in humans.  HIV-2 is uncommon, occurring almost exclusively in western Africa.  It is most closely related to, and thought to have evolved from, a SIV specific to the Sooty mangabey (Cercocebus atys), an endangered primate endemic to forests of western Africa.  HIV-2 is less aggressive in causing disease, and is not transmitted from host to host as easily as HIV-1 (AVERT 2014).

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