HIV/AIDS

About HIV/AIDS

The Virus

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 and apes, sheep and goats. 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. Of the many different strains of SIV found in various monkey and ape species in Africa, some causing an AIDS-like disease in their host, the most similar to HIV-1 is the chimpanzees SIV. 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 an 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.

HIV itself has numerous varieties and has been shown to mutate even within an individual during the progress of the infection. AIDS develops in an 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 late 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 most countries, in its early stages the viral epidemic progressed undetected.

Transmission

HIV is easily killed outside the human body and therefore can only be transmitted directly from person to person, either by sexual contact, exchange of blood or body fluids or from mother to child. Sexual transmission of the HIV is relatively inefficient and repeated unprotected exposures are normally required. HIV incidence and prevalence can vary greatly from country to country and even within countries, depending on several risk factors and risk behaviours. Since the HIV epidemic is driven mainly by sexual transmission, the level and intensity of risk behaviours (vaginal or anal unprotected sex) in a given community are the main determinants of the spread of the virus.

Related Link

Adapted from "Programme for the Surveillance of HIV/AIDS and Sexually Transmitted Infections".

Global Health Sector Strategy on HIV/AIDS 2011-2015: Let’s do what’s right for everyone

Global Hepatitis Programme

Contact us

World Health Organization
HIV/AIDS Department
20, Avenue Appia
CH-1211 Geneva 27
Switzerland
Email: hiv-aids@who.int