The cause is a virus (called the human immunodeficiency virus or HIV) transmitted in blood and in sexual fluids, and though the incubation period may be long the fully developed disease is invariably fatal. Aids was first identified in the early 1980s and now affects millions of people. In the developed world the disease first spread among homosexuals, intravenous drug users, and recipients of infected blood transfusions, before reaching the wider population. This has tended to overshadow a greater epidemic in parts of Africa, where transmission is mainly through heterosexual contact.