Global Alert and Response (GAR)

WHO Report on Global Surveillance of Epidemic-prone Infectious Diseases - Human Immunodeficiency Virus and Acquired Immune Deficiency Syndrome (HIV/AIDS)

Background of the disease

AIDS (Acquired Immune Deficiency Syndrome) is caused by a virus, HIV (Human Immunodeficiency Virus) first isolated in 1983. It has been identified in over 200 countries and territories worldwide and is spreading rapidly in many affected populations, particularly in developing countries.

HIV belongs to an unusual group of viruses called retroviruses, which include viruses causing leukaemia in humans, cats, cattle and other animals, and certain other viruses found in monkeys, apes, sheep and pigs. Retroviruses also belong to a subgroup called lentiviruses, because they are slow to cause disease.

There are two main strains of HIV: HIV-1 that has caused the majority of infections and AIDS cases and HIV-2, which is concentrated in selected countries. Of the other known related viruses, a type of retrovirus found in many other primates (Simian Immunodeficiency Virus, SIV) may be the most likely contender for the origin of HIV. Different strains of SIV have been found in various monkey and ape species in Africa, and some cause an AIDS-like disease in their host. One of the most similar to HIV-1, however, is the SIV found in chimpanzees. Many viruses mutate, or change, more easily than more complex organisms. HIV itself has numerous varieties and has been shown to mutate even within an individual during the progress of the infection. AIDS develops in a HIV-positive person after years of infection, as HIV steadily weakens the body's immune system and increases its vulnerability to pneumonia, tuberculosis, diarrhoea, tumours and other opportunistic illnesses. With the number of people infected with HIV continuing to rise, the number of people falling sick and dying of AIDS will multiply.

While the origins of AIDS remain obscure, it is known that HIV occurred as long ago as the 1950s in isolated individuals. It began to be widespread in the mid- to late-1970s but, because of the long incubation period, the virus did not cause widespread disease until the 1980s. In its early stages the viral epidemic progresses unseen.

By the 1990s, however, AIDS itself reached epidemic proportions in many countries. Of the estimated 34.3 million people living with HIV at the end of 1999, 24.5 million live in sub-Saharan Africa, the hardest hit region. The estimated percentage of adults living with HIV reaches up to 26% in some countries in this region (Map 9.1). AIDS has become the main cause of death in parts of Africa and is responsible for the majority of adult hospital admissions in some cities. Many AIDS patients are never diagnosed, and their deaths may be attributed to other causes.