An example of a retrovirus is human immunodeficiency virus (HIV), which causes acquired immune deficiency syndrome (AIDS). AIDS was first recognized in 1982, when a number of homosexual males in the United States began to exhibit symptoms of a new immune system–deficiency disease. In that year, Robert Gallo proposed that AIDS was caused by a retrovirus. Between 1983 and 1984, as the AIDS epidemic became widespread, the HIV retrovirus was isolated from people with the disease. AIDS is now known to be caused by two different immunodeficiency viruses, HIV-1 and HIV-2, which together have infected more than 60 million people worldwide. Of those infected, 30 million have died. Most cases of AIDS are caused by HIV-1, which now has a global distribution; HIV-2 is primarily found in western Africa.

HIV illustrates the importance of genetic recombination in viral evolution. Studies of the DNA sequences of HIV and other retroviruses reveal that HIV-1 is closely related to the simian immunodeficiency virus found in chimpanzees (SIV_cpz). Many wild chimpanzees in Africa are infected with SIV_cpz, although it doesn’t cause AIDS-like symptoms in these animals. SIV_cpz is itself a hybrid that resulted from recombination between a retrovirus found in the red-capped mangabey (a monkey) and a retrovirus found in the greater spot-nosed monkey (Figure 7.27). Apparently, one or more chimpanzees became infected with both viruses; recombination between the viruses produced SIV_cpz, which was then transmitted to humans through contact with infected chimpanzees. In humans, SIV_cpz underwent significant evolution to become HIV-1, which then spread throughout the world to produce the AIDS epidemic. Several independent transfers of SIV_cpz to humans gave rise to different strains of HIV-1. HIV-2 evolved from a different retrovirus, SIV_sm, found in sooty mangabeys.

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HIV is transmitted by sexual contact between humans and through any type of blood-to-blood contact, such as that caused by the sharing of dirty needles by drug users. It can also be transmitted between mother and child during pregnancy and after pregnancy in breast milk. Until screening tests were developed to identify HIV-infected blood, transfusions and clotting factors used to treat hemophilia were sources of infection as well.

HIV principally attacks a class of blood cells called helper T lymphocytes, or simply helper T cells (Figure 7.28). HIV enters a helper T cell, undergoes reverse transcription, and integrates into the chromosome. The virus reproduces rapidly, destroying the T cell as new virus particles escape from the cell. Because helper T cells are central to immune function, people with AIDS have a diminished immune response; most people with the disease die of secondary infections that develop because they have lost the ability to fight off pathogens.

The HIV genome is 9749 nucleotides long and carries gag, pol, env, and six other genes that regulate the life cycle of the virus. HIV’s reverse transcriptase is very error prone, giving the virus a high mutation rate and allowing it to evolve rapidly, even within a single host. This rapid evolution makes the development of an effective vaccine against HIV particularly difficult. Genetic variation within the human population also affects the virus. To date, more than 10 loci in humans that affect HIV infection and the progression of AIDS have been identified.