AIDS is the final, life-threatening stage of infection with human immunodeficiency virus (HIV). AIDS stands for acquired immune deficiency syndrome. The name refers to the fact that HIV severely damages the immune system, the body's most important defence against disease. Cases of AIDS were first identified in 1981, in the United States, but researchers have detected HIV in a specimen collected in 1959 in central Africa. Millions of AIDS cases have been diagnosed worldwide. 

  AIDS virus in a white blood cell 
How AIDS affect the body : AIDS is caused by two viruses that belong to a group called retroviruses. The first AIDS virus was isolated by researchers in France in 1983 and in the United States in 1984. The virus became known as HIV-1. In 1985, scientists in France identified another closely related virus that also produces AIDS. This virus, named HIV-2, occurs mainly in Africa. HIV-1 occurs throughout the world. 

HIV infects certain white blood cells, including T-helper cells and macrophages, that play key roles in the immune system. The virus attaches to CD4 receptor molecules on the surface of these cells, which are often called CD4 cells. HIV enters CD4 cells and inserts its own genes into the cell's reproductive system. The cell then produces more HIV, which spreads to other CD4 cells. Eventually infected cells die. 

Symptoms. People infected with HIV eventually develop symptoms that may be caused by other, less serious conditions. With HIV infection, however, these symptoms are prolonged and often more severe. They include enlarged lymph glands, tiredness, fever, loss of appetite and weight, diarrhoea, yeast infections of the mouth and vagina, and night sweats. 

HIV commonly causes a severe "wasting syndrome," resulting in substantial weight loss, a general decline in health, and eventual death. In many patients, the virus infects the brain and nervous system, and may cause dementia, a condition characterized by sensory, thinking, or memory disorders. HIV infection of the brain may also cause movement or coordination problems. 

Opportunistic infections. HIV makes infected people susceptible to illnesses that do not normally occur or that are normally not serious. These infections are called opportunistic because they take advantage of damage to the immune system. With the onset of an opportunistic infection or one of several other severe illnesses or with a marked decline in the number of CD4 cells, an HIV-infected person is considered to have AIDS. 

There are many opportunistic illnesses that typically affect AIDS patients. In North America and Europe, Pneumocystis carinii pneumonia, yeast infections of the oesophagus (tube that carries food to the stomach), cytomegalovirus retinitis, Kaposi's sarcoma, and tuberculosis are the most common. People with AIDS may contract several of these diseases. 

Pneumocystis carinii pneumonia, which is an infection of the lungs, is the leading cause of death among AIDS patients. Yeast infections of the oesophagus cause severe pain when swallowing and result in weight loss and dehydration. Cytomegalovirus retinitis is an eye infection that can cause blindness. Kaposi's sarcoma is a form of cancer that usually arises in the skin. The tumours may look like bruises, but they grow. 

Another illness that defines AIDS in HIV-infected people is tuberculosis. For many decades, the number of cases of tuberculosis in the United States declined. However, in the mid-1980's, doctors noticed a growing number of cases of tuberculosis in HIV patients. People with HIV are especially vulnerable to tuberculosis because of their damaged immune systems. 

An HIV-infected person may develop AIDS from 2 to 15 or more years after becoming infected. In children born with HIV infection, this interval is usually shorter. Medical treatment can increase the interval by inhibiting the growth of HIV, preserving the immune system, and delaying the onset of opportunistic illnesses. A few people who have been infected with HIV for more than 12 years have not developed any symptoms or suffer only minor symptoms. Others have symptoms of HIV infection but none of the opportunistic illnesses. An infected person can transmit the virus to another person whether or not symptoms are present. Infection with HIV appears to be lifelong in all who become infected.