HIV is not transmitted by casual contact; transmission requires a direct exchange of body fluids, such as blood or blood products, breast milk, semen, or vaginal secretions, most commonly as a result of sexual activity or the sharing of needles among drug users. Such a transmission may also occur from mother to baby during pregnancy or at birth. Saliva, tears, urine, feces, and sweat do not appear to transmit the virus. Since 2010 several studies have shown that transmission of HIV is significantly reduced to individuals who take antiretroviral drugs prophylactically, and in 2012 the Food and Drug Administration approved a pill that combines two antiretroviral drugs, tenofovir and emtricitabine, for use in preventing HIV infection.
By 2012 it was estimated that as many as 34 million people were infected with HIV worldwide, the great majority in Third World countries; some 30 million had died from AIDS. The disease in sub-Saharan Africa, which has been especially hard hit, in the main has been transmitted heterosexually and has been exacerbated by civil wars and refugee problems and less restrictive local mores with regard to sex. Some 22.5 million people were infected with HIV in this region, where, in many countries, the prevalence of AIDS has lowered the life expectancy. Nonetheless, the spread of the disease had slowed somewhat during the previous decade; an estimated 3.2 million new HIV infections occurred in 2001, but only 2.5 million in 2011.
In the United States, the demographics of AIDS have changed over time. In the 1980s it was seen mainly in homosexual and bisexual men and was one of the spurs to the gay-rights movement, as activists lobbied for research and treatment monies and began education and prevention programs. Also in the early years, before careful screening of blood products was deemed necessary, the virus was contracted by an estimated 9,000 hemophiliacs (see hemophilia), and a small number of people were infected by surgical or emergency blood transfusions. Before long, however, the majority of new HIV infections were seen in drug users who contracted the disease from shared needles or unprotected sex; a large proportion of infected women were drug users or partners of drug users. Nearly a third of the infants born to HIV-infected women are infected with the virus. (Some of these infants test positive for AIDS only because of the mother's antibodies and later test negative.) In the early 21st cent., however, the majority of new cases, which were estimated to average 50,000 per year in 2006–9, were again in homosexual and bisexual men.