AIDS is thought to have originated in Africa

AIDS is thought to have originated in Africa, where monkeys and apes harbor a virus similar to HIV called SIV (simian immunodeficiency virus). Scientists believe the illness first jumped to humans from wild chimpanzees in central Africa.

How the disease crossed the species barrier remains a puzzle. The leading theory is that it was picked up by people who hunted or ate infected chimpanzees. Researchers have dated the virus in humans to about 1930 using scientific estimates of the time it's taken for different strains of HIV to evolve.

AIDS today is a global pandemic affecting every country. In 2006, an estimated 39.5 million people had HIV/AIDS. Almost three million of them died.

The region most devastated by the disease is sub-Saharan Africa. It accounts for two-thirds of the world's HIV cases and nearly 75 percent of deaths due to AIDS. Infection rates vary, with southern African countries worst affected. In South Africa, an estimated 29 percent of pregnant women have HIV. Infection rates in Zimbabwe's adult population exceed 20 percent, while in Swaziland a third of adults are HIV-positive. Poverty, inadequate health care and education, and promiscuity have all been highlighted to explain Africa's AIDS nightmare.