Over time, they cause acquired immunodeficiency syndrome (AIDS),[1][2] a condition in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive.

[4] In most cases, HIV is a sexually transmitted infection and occurs by contact with or transfer of blood, pre-ejaculate, semen, and vaginal fluids.

[5][6] Non-sexual transmission can occur from an infected mother to her infant during pregnancy, during childbirth by exposure to her blood or vaginal fluid, and through breast milk.

Research has shown (for both same-sex and opposite-sex couples) that HIV is not contagious during sexual intercourse without a condom if the HIV-positive partner has a consistently undetectable viral load.

[14] When CD4+ T cell numbers decline below a critical level, cell-mediated immunity is lost, and the body becomes progressively more susceptible to opportunistic infections, leading to the development of AIDS.

[25] The single-stranded RNA is tightly bound to nucleocapsid proteins, p7, and enzymes needed for the development of the virion such as reverse transcriptase, proteases, ribonuclease and integrase.

This is one of the most densely glycosylated molecules known and the density is sufficiently high to prevent the normal maturation process of glycans during biogenesis in the endoplasmic and Golgi apparatus.

[31] The unusual processing and high density means that almost all broadly neutralising antibodies that have so far been identified (from a subset of patients that have been infected for many months to years) bind to, or are adapted to cope with, these envelope glycans.

In the tonsils and adenoids of HIV-infected patients, macrophages fuse into multinucleated giant cells that produce huge amounts of virus.

[54] These variants then replicate more aggressively with heightened virulence that causes rapid T cell depletion, immune system collapse, and opportunistic infections that mark the advent of AIDS.

A number of studies with subtype B-infected individuals have determined that between 40 and 50 percent of AIDS patients can harbour viruses of the SI and, it is presumed, the X4 phenotypes.

[59][failed verification] During the microtubule-based transport to the nucleus, the viral single-strand RNA genome is transcribed into double-strand DNA, which is then integrated into a host chromosome.

[69] The process of reverse transcription is extremely error-prone, and the resulting mutations may cause drug resistance or allow the virus to evade the body's immune system.

At the same time, certain guanosine residues in the gRNA are made available for binding of the nucleocapsid (NC) protein leading to the subsequent virion assembly.

[84] Bonhoeffer et al.[85] suggested that template switching by reverse transcriptase acts as a repair process to deal with breaks in the single-stranded RNA genome.

For HIV, as well as for viruses in general, successful infection depends on overcoming host defense strategies that often include production of genome-damaging reactive oxygen species.

The Gag (p55) and Gag-Pol (p160) polyproteins also associate with the inner surface of the plasma membrane along with the HIV genomic RNA as the forming virion begins to bud from the host cell.

As this happens, the reverse transcriptase, by jumping back and forth between the two different RNA templates, will generate a newly synthesized retroviral DNA sequence that is a recombinant between the two parental genomes.

SIV strains of the African green monkey (SIVagm) and sooty mangabey (SIVsmm) are thought to have a long evolutionary history with their hosts.

Nef's function in non-pathogenic forms of SIV is to downregulate expression of inflammatory cytokines, MHC-1, and signals that affect T cell trafficking.

[19][124] While HIV is highly virulent, transmission through sexual contact does not occur when an HIV-positive individual maintains a consistently undetectable viral load (<50 copies/ml) due to antiretroviral treatment.

Across these four studies, a total of 4,097 couples participated from four continents, reporting 151,880 acts of condomless sex with zero phylogenetically-linked HIV transmissions when the positive partner had an undetectable viral load.

[134] Following these findings, the U=U consensus statement advocating the use of the term 'zero risk' was endorsed by numerous individuals and organizations, including the CDC, the British HIV Association, and The Lancet medical journal.

[140] Soon thereafter, researchers at the NYU School of Medicine studied gay men developing a previously rare skin cancer called Kaposi's sarcoma (KS).

[148] 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.

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 primary HIV infection.

The ARV strains indicated, for the first time, the heterogeneity of HIV isolates and several of these remain classic examples of the AIDS virus found in the United States.

[168] While transmission rates of HIV during vaginal intercourse are typically low, they are increased manyfold if one of the partners has a sexually transmitted infection resulting in genital ulcers.

[170] An alternative and likely complementary hypothesis points to the widespread use of 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.

This observation is consistent with both of the two prevailing views of the origin of the HIV epidemics, namely SIV transmission to humans during the slaughter or butchering of infected primates, and the colonial expansion of sub-Saharan African cities.