The acquired immunodeficiency syndrome (AIDS) was first recognized in 1981 and has since become a major worldwide pandemic. AIDS is caused by the human immunodeficiency virus (HIV). By leading to the destruction and/or functional impairment of cells of the immune system, notably CD4+ T cells, HIV progressively destroys the body's ability to fight infections and certain cancers.

An HIV-infected person is diagnosed with AIDS when his or her immune system is seriously compromised and manifestations of HIV infection are severe. The U.S. Centers for Disease Control and Prevention (CDC) currently defines AIDS in an adult or adolescent age 13 years or older as the presence of one of 26 conditions indicative of severe immunosuppression associated with HIV infection, such as Pneumocystis carinii pneumonia (PCP), a condition extraordinarily rare in people without HIV infection. Most other AIDS-defining conditions are also "opportunistic infections" which rarely cause harm in healthy individuals. A diagnosis of AIDS also is given to HIV-infected individuals when their CD4+ T-cell count falls below 200 cells/cubic millimeter (mm³) of blood. Healthy adults usually have CD4+ T-cell counts of 600-1,500/mm³ of blood. In HIV-infected children younger than 13 years, the CDC definition of AIDS is similar to that in adolescents and adults, except for the addition of certain infections commonly seen in pediatric patients with HIV. (CDC. MMWR 1992;41(RR-17):1; CDC. MMWR 1994;43(RR-12):1).

In many developing countries, where diagnostic facilities may be minimal, healthcare workers use a World Health Organization (WHO) AIDS case definiton based on the presence of clinical signs associated with immune deficiency and the exclusion of other known causes of immunosuppression, such as cancer or malnutrition. An expanded WHO AIDS case definition, with a broader spectrum of clinical manifestations of HIV infection, is employed in settings where HIV antibody tests are available (WHO. Wkly Epidemiol Rec. 1994;69:273).

As of the end of 2000, an estimated 36.1 million people worldwide - 34.7 million adults and 1.4 million children younger than 15 years - were living with HIV/AIDS. Through 2000, cumulative HIV/AIDS-associated deaths worldwide numbered approximately 21.8 million - 17.5 million adults and 4.3 million children younger than 15 years. In the United States, an estimated 800,000 to 900,000 people are living with HIV infection. As of December 31, 1999, 733,374 cases of AIDS and 430,441 AIDS-related deaths had been reported to the CDC. AIDS is the fifth leading cause of death among all adults aged 25 to 44 in the United States. Among African-Americans in the 25 to 44 age group, AIDS is the leading cause of death for men and the second leading cause of death for women (UNAIDS. AIDS epidemic update: December 2000; CDC. HIV/AIDS Surveillance Report 1999;11[2]:1; CDC. MMWR 1999;48[RR13]:1).

This document summarizes the abundant evidence that HIV causes AIDS. Questions and answers at the end of this document address the specific claims of those who assert that HIV is not the cause of AIDS.