HIV is a retrovirus that causes AIDS by infecting helper T-cells of the immune system. There are two types of HIV: HIV-1 and HIV-2. HIV-1 is responsible for the vast majority of AIDS in the United States. HIV-2, seen more often in western Africa, has a slower course than HIV-1. There are many strains of both types and the virus mutates rapidly, a trait that has made it especially difficult for researchers to find an effective treatment or vaccine. In many cases, a person's immune system will fight of the invasion of HIV for many years, producing billions of CD4 cells daily, always trying to keep up with the HIV's mutations, before it succumbs and permits the well-known sign of AIDS to develop.
HIV is especially lethal because it attacks the very immune system cells (variously called T4, CD4, or T-helper lymphocytes) that would ordinarily fight off such a viral infection. Receptors on these cells appear to enable the viral RNA to enter the cell. As with all retro viruses, once the RNA is inside the cell, an enzyme celled reverse transcripts allows it to act as the template for its own RNA to DNA transcription. The resultant viral DNA inserts itself into a cell's DNA and is reproduced along with the cell and its daughters.
AIDS stands for acquired immunodeficiency syndrome, which is fatal disease caused by a rapidly mutating retrovirus that attacks the immune system and leaves the victim vulnerable to infections, malignancies, and neurological disorders. It was first recognized as a disease in 1981. The virus as isolated in 1983 and was ultimately named the human immunodeficiency virus (HIV). In 1999 an international team of genetic scientists reported that HIV-1 can be traced to a closely related strain of virus, called simian immunodeficiency virus (SIV). SIV is found in chimpanzees.