AIDS was first recognized as a distinct clinical entity in 1981. In retrospect, however, isolated cases appear to have occurred during the 1970s, and even earlier in several areas (Africa, Europe, Haiti, USA). Of the estimated 33 million persons (95% confidence interval, 31–36 million) worldwide living with HIV infection or AIDS (HIV/AIDS) in 2007, the largest elements were estimated at 22.5 million in sub-Saharan Africa, 4.0 million in south and southeastern Asia, 1.6 million in Latin America, 1.6 million in eastern Europe and central Asia, and 1.3 million in North America. Globally, AIDS caused an estimated 2.1 million deaths in 2007 (1.9–2.4 million); the epidemic has continued growing, with estimates of 2.5 million new infections (1.8–4.1 million) and 2.1 million children under 15 years (1.9–2.4 million) living with HIV/AIDS. AIDS is now the leading cause of death in persons aged 15 to 59 years old. HIV-1 is the most prevalent HIV species throughout the world; HIV-2 has been found primarily in western Africa, with cases also in countries linked epidemiologically to western Africa.

Sub-Saharan Africa is the most affected region; southern Africa is the worst affected. Unlike other regions, the majority of people (61%) living with HIV in sub-Saharan Africa are women. Prevalence has stabilized or is decreasing in most countries, along with some evidence of decreasing risk behavior. In Asia, prevalence is highest in southeast Asia, with considerable variation in trends – declining in some countries while continuing to grow in others. The Caribbean region has the second-highest prevalence worldwide after sub-Saharan Africa, with two countries, the Dominican Republic and Haiti, accounting for most people with HIV infection in the region. In eastern Europe and central Asia, Russia and Ukraine account for most new HIV diagnoses, but rates are also rising in other countries. Injection drug use is a major factor in the region. Latin America's HIV epidemics are generally stable, with transmission largely in higher-risk populations such as sex workers and men who have sex with men. In North America, Western, Central Europe, Australia and New Zealand, HIV continues to be transmitted mainly through unprotected sex between men.