HIV

HIV, the human immunodeficiency virus, is a retrovirus that attacks the immune system, especially the CD4 helper T cells that coordinate immune responses. Untreated infection can progress over years to acquired immunodeficiency syndrome (AIDS), in which the body can no longer defend itself against many infections and cancers.

HIV particles carry two copies of a single-stranded RNA genome of about 9,700 bases inside a protein capsid and a lipid envelope. After entering a cell, the viral enzyme reverse transcriptase copies the RNA into DNA, which is inserted into the host's chromosome by integrase. A viral protease later cuts long precursor proteins into their working pieces so new virus particles can mature. Two types exist: HIV-1, which is global, and the less transmissible HIV-2, concentrated in West Africa.

HIV-1 arose from simian immunodeficiency viruses of chimpanzees that crossed into humans in central Africa in the early twentieth century. The virus was identified in 1983 by the laboratories of Françoise Barré-Sinoussi and Luc Montagnier, who shared the 2008 Nobel Prize with Harald zur Hausen.

Combination antiretroviral therapy, introduced in the mid-1990s, targets several viral enzymes and can suppress the virus to undetectable levels, which keeps people healthy and prevents sexual transmission. Pre-exposure prophylaxis protects people at risk. About 40 million people live with HIV worldwide, and no vaccine is available, partly because the virus mutates rapidly and hides in dormant cells. HIV is an example of how the genetic code of a virus drives its behavior, and it is monitored as part of public health.

• Retroviruses — the virus family it belongs to
• Reverse Transcriptase — the enzyme it uses to copy its genome
• Vaccine — prevention that does not yet exist for HIV