Discovery
AIDS was first clinically observed in 1981 in the United States.[95] The initial cases were a cluster of injection drug users and gay men with no known cause of impaired immunity who showed symptoms of Pneumocystis carinii pneumonia (PCP), a rare opportunistic infection that was known to occur in people with very compromised immune systems.[96]Soon thereafter, additional gay men developed a previously rare skin cancer called Kaposi's sarcoma (KS).[97][98] Many more cases of PCP and KS emerged, alerting U.S.Centers for Disease Control and Prevention (CDC) and a CDC task force was formed to monitor the outbreak.[99]
In the beginning, the CDC did not have an official name for the disease, often referring to it by way of the diseases that were associated with it, for example, lymphadenopathy, the disease after which the discoverers of HIV originally named the virus.[100][101] They also used Kaposi's Sarcoma and Opportunistic Infections, the name by which a task force had been set up in 1981.[102] In the general press, the term GRID, which stood for gay-related immune deficiency, had been coined.[103] The CDC, in search of a name, and looking at the infected communities coined "the 4H disease," as it seemed to single out homosexuals, heroin users, hemophiliacs, and Haitians.[104][105] However, after determining that AIDS was not isolated to the gay community,[102] it was realized that the term GRID was misleading and AIDS was introduced at a meeting in July 1982.[106] By September 1982 the CDC started using the name AIDS.[107]
 
Robert Gallo, co-discoverer of HIV
In 1983, two separate research groups led by Robert Gallo and Luc Montagnier independently declared that a novel retrovirus may have been infecting AIDS patients, and published their findings in the same issue of the journal Science.[108][109] Gallo claimed that a virus his group had isolated from an AIDS patient was strikingly similar in shape to other human T-lymphotropic viruses (HTLVs) his group had been the first to isolate. Gallo's group called their newly isolated virus HTLV-III. At the same time, Montagnier's group isolated a virus from a patient presenting with swelling of the lymph nodes of the neck and physical weakness, two classic symptoms of AIDS. Contradicting the report from Gallo's group, Montagnier and his colleagues showed that core proteins of this virus were immunologically different from those of HTLV-I. Montagnier's group named their isolated virus lymphadenopathy-associated virus (LAV).[99] As these two viruses turned out to be the same, in 1986, LAV and HTLV-III were renamed HIV.[110]
Origins
Both HIV-1 and HIV-2 are believed to have originated in non-human primates in West-central Africa, and are believed to have transferred to humans (a process known aszoonosis) in the early 20th Century.[111][112]
HIV-1 appears to have originated in southern Cameroon through the evolution of SIV(cpz), a simian immunodeficiency virus (SIV) that infects wild chimpanzees (HIV-1 descends from the SIV(cpz) endemic in the chimpanzee subspecies Pan troglodytes troglodytes).[113][114] The closest relative of HIV-2 is SIV (smm), a virus of the sooty mangabey(Cercocebus atys atys), an old world monkey living in litoral West Africa (from southern Senegal to western Côte d'Ivoire).[12] New World monkeys such as the owl monkey are resistant to HIV-1 infection, possibly because of a genomic fusion of two viral resistance genes.[115] HIV-1 is thought to have jumped the species barrier on at least three separate occasions, giving rise to the three groups of the virus, M, N, and O.[116]
 
Left to right: the African green monkey source of SIV, the sooty mangabey source of HIV-2, and the chimpanzee source of HIV-1
There is evidence that humans who participate in bushmeat activities, either as hunters or as bushmeat vendors, commonly acquire SIV.[117] However, SIV is a weak virus, and it is typically suppressed by the human immune system within weeks of infection. It is thought that several transmissions of the virus from individual to individual in quick succession are necessary to allow it enough time to mutate into HIV.[118]Furthermore, due to its relatively low person-to-person transmission rate, it can only spread throughout the population in the presence of one or more of high-risk transmission channels, which are thought to have been absent in Africa prior to the 20th century.
Specific proposed high-risk transmission channels, allowing the virus to adapt to humans and spread throughout the society, depend on the proposed timing of the animal-to-human crossing. Genetic studies of the virus suggest that the most recent common ancestor of the HIV-1 M group dates back to circa 1910.[119]Proponents of this dating link the HIV epidemic with the emergence of colonialism and growth of large colonial African cities, leading to social changes, including a higher degree of sexual promiscuity, the spread ofprostitution, and the concomitant high frequency of genital ulcer diseases (such as syphilis) in nascent colonial cities.[120] While transmission rates of HIV during vaginal intercourse are typically low, they are increased many fold if one of the partners suffers from a sexually transmitted infection resulting in genital ulcers. Early 1900s colonial cities were notable due to their high prevalence of prostitution and genital ulcers to the degree that as of 1928 as many as 45% of female residents of eastern Leopoldville were thought to have been prostitutes and as of 1933 around 15% of all residents of the same city were infected by one of the forms of syphilis.[120]
An alternative view holds that unsafe medical practices in Africa during years following World War II, such as unsterile reuse of single use syringes during mass vaccination, antibiotic, and anti-malaria treatment campaigns, were the initial vector that allowed the virus to adapt to humans and spread.[118][121][122]
The earliest well documented case of HIV in a human dates back to 1959 in the Belgian Congo.[123] The virus may have been present in the United States as early as the mid-to-late 1950s, as a sixteen-year-old male presented with symptoms in 1966 died in 1969.[124]