A new study finds that HIV/AIDS originated in the 1920s in the central African city of Kinshasa, in what is now the Democratic Republic of Congo.
Researchers say "a perfect storm" of factors "primed the HIV pandemic," including rail and river transport in 1960s Congo, a booming sex trade and the use of unsterilized needles in health clinics.
The international research team traced HIV's genetic history using archived samples of the virus's genetic code.
HIV was first recognized in the 1980s. Worldwide, some 75 million people have been infected with it. Around 36 million of them have died.
HIV gets its letters from its official name, human immunodeficiency virus, a mutation of simian immunodeficiency virus, which is found in chimpanzees.
The researchers believe the virus first jumped to humans through infected blood from bush meat.
From there, the virus spread to areas of Africa beyond Kinshasa, and eventually to the world. The report establishes that more than one million people were using Kinshasa's Belgium-backed railways by the end of 1940, contributing to the disease's spread. At that time, Kinshasa was part of the Belgian Congo.