The first AIDS (acquired immunodeficiency syndrome) cases were identified in 1981 by physicians in Los Angeles and New York City. Since that time researchers traced possible cases of the disease back to 1969. The human immunodeficiency virus (HIV), which severely damages the body’s ability to fight disease, is transmitted through sexual contact, shared drug needles, and infected blood transfusions. While the disease was believed to have been transmitted somehow to humans from monkeys (since research shows HIV to be similar to simian immunodeficiency viruses), HIV has never been isolated in any wild animal. While the source of the deadly disease has not been definitively determined, scientists believe that infection began in Africa during the 1960s and 1970s when significant numbers of people migrated from rural areas to cities. The overcrowding and unemployment that resulted contributed to the spread of sexually transmitted diseases.

AIDS is now considered endemic to many developing nations, where it is spread mostly among heterosexual men and women. In developed nations, education programs have made the public aware of how the disease is transmitted, helping curb the spread of HIV. Drug treatments are still being developed to treat HIV/AIDS; no cure has been discovered. A 2004 report from the United Nations stated that there were 38 million people living with HIV in the world, almost 70 percent of them in sub-Saharan Africa. The same report states that more than 20 million people had died since the first cases were identified in 1981.