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HIV, human immunodeficiency virus, either of two closely related retroviruses that invade T-helper lymphocytes and are responsible for AIDS. 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 off 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 signs 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 retroviruses, once the RNA is inside the cell, an enzyme called reverse transcriptase 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. In 2012 the Food and Drug Administration approved a pill that combines two antiretroviral drugs, tenofovir and emtricitabine, for use in preventing infection with HIV, and two years later the Centers for Disease Control and Prevention called for the drug combination to be prescribed to uninfected patients who are at risk for AIDS in an effort to reduce number of new HIV infections.

The exact origin of the virus in humans is unclear. Scientists surmise that HIV-1 jumped from African chimpanzees, who harbor a similar strain of SIV (simian immunodeficiency virus), to humans via the butchering of meat or an animal bite. The first case documented in humans dates from 1959, but genetic analysis published in 2008 estimated that it originated some time between 1884 and 1924. A 2014 analysis suggested that the most widespread form of HIV-1 in humans originated in 1920s in what was then the Belgian Congo. AIDS is believed to have spread to the Caribbean in the 1960s and the United States in the early 1970s. The virus was isolated by Luc Montagnier of France's Pasteur Institute in 1983. It went through several name changes before the official name, human immunodeficiency virus, was agreed upon.

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McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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