AIDS

AIDS wasn't discovered until the early 1980s, when doctors in the United States noticed clusters of patients suffering from highly unusual diseases. First seen in gay men in New York and California, these illnesses included Kaposi's sarcoma, a rare skin cancer, and a type of lung infection carried by birds.

Soon cases were also detected in intravenous drug users and recipients of blood transfusions. By 1982 the illness had a name—acquired immune deficiency syndrome. AIDS has since killed around 25 million people worldwide, orphaning 12 million children in Africa alone.

AIDS is triggered by a virus acquired through direct contact with infected body fluids. The virus causes an immune deficiency by attacking a type of white blood cell that helps to fight infections. Because this leads to various diseases, not a single illness, AIDS is referred to as a syndrome. The virus is called HIV (human immunodeficiency virus).

Unprotected sex is HIV's main route into humans, where it targets the white blood cell known as CD4. The virus replicates inside, eventually bursting out and flooding the body in the billions. The immune system then kicks in, and the body and the virus wage all-out war. During the height of battle billions of CD4 cells can be destroyed in a single day. As the cell count drops, the immune system begins to fail and opportunistic infections such as tuberculosis take hold.

Ape Origins

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.

Treatments But No Cure

Efforts to prevent the spread of AIDS focus on sex education and the use of condoms. Other measures, such as male circumcision, may also help to cut the risk of sexually transmitted infection.

There is no cure for AIDS, but treatments are available that combat its onset. Antiviral drugs work by slowing the replication of HIV in the body. These drugs need to be used in combination because the virus readily mutates, creating new and often drug-resistant strains. Such treatments are expensive, however, and are still denied to millions of people in the developing world.

In the future, the hope is for an AIDS vaccine that would prevent HIV infection. Researchers are currently working on more than 30 potential candidates.