An individual can become infected with HIV-1 from many sources, including contaminated blood and blood products (such as though medical injections, blood transfusions, and injection drug usage [IDU]), an infected mother transmitting the virus to her baby (before, during, or after birth and through breast milk), and through either vaginal or anal intercourse. The relative importance of a source of HIV-1 varies in different parts of the world. In some countries, infection with HIV-1 is mainly detected in specific groups at risk, including MSM, injecting drug users, sex workers, and the regular partners of such persons. In most countries of sub-Saharan Africa, however, HIV-1 is self-sustaining in the general population through heterosexual intercourse in HIV-serodiscordant couples. MTCT, mother-to-child transmission. The data presented here were mostly generated for 2005, but some data are from earlier years. Reproduced with permission from the Global HIV Prevention Working Group (103).