Cause

For more details on this topic, see HIV.

Scanning electron micrograph of HIV-1, colored green, budding from a cultured lymphocyte.

A generalized graph of the relationship between HIV copies (viral load) and CD4 counts over the average course of untreated HIV infection; any particular individual's disease course may vary considerably.                      CD4⁺ T Lymphocyte count (cells/mm³)                      HIV RNA copies per mL of plasma

AIDS is the ultimate clinical consequence of infection with HIV. HIV is a retrovirus that primarily infects vital organs of the human immune system such as CD4⁺ T cells (a subset of T cells), macrophages and dendritic cells. It directly and indirectly destroys CD4⁺ T cells.^[58]
Once HIV has killed so many CD4⁺ T cells that there are fewer than 200 of these cells per microliter (µL) of blood, cellular immunity is lost. Acute HIV infection usually progresses over time to clinical latent HIV infection and then to early symptomatic HIV infection and later to AIDS, which is identified either on the basis of the amount of CD4⁺ T cells remaining in the blood, and/or the presence of certain infections, as noted above.^[59]
In the absence of antiretroviral therapy, the median time of progression from HIV infection to AIDS is nine to ten years, and the median survival time after developing AIDS is only 9.2 months.^[60] However, the rate of clinical disease progression varies widely between individuals, from two weeks up to 20 years.
Many factors affect the rate of progression. These include factors that influence the body's ability to defend against HIV such as the infected person's general immune function.^[61][62] Older people have weaker immune systems, and therefore have a greater risk of rapid disease progression than younger people.
Poor access to health care and the existence of coexisting infections such as tuberculosis also may predispose people to faster disease progression.^[60][63][64] The infected person's genetic inheritance plays an important role and some people are resistant to certain strains of HIV. An example of this is people with the homozygous CCR5-Δ32 variation are resistant to infection with certain strains of HIV.^[65] HIV is genetically variable and exists as different strains, which cause different rates of clinical disease progression.^[66][67][68]
There are a number HIV and AIDS misconceptions. Three of the most common are that AIDS can spread through casual contact, that sexual intercourse with a virgin will cure AIDS, and that HIV can infect only homosexual men and drug users. Other misconceptions are that any act of anal intercourse between gay men can lead to AIDS infection, and that open discussion of homosexuality and HIV in schools will lead to increased rates of homosexuality and AIDS.^[69][70]