HIV/AIDS originated in Kinshasa in 1920, long before it was officially recognized in the 1980s, according to a study published on Friday in the journal Science.

The authors of the study traced the origin of the disease to Kinshasa, which is now part of the Democratic Republic of Congo, using archived samples of HIV’s genetic code. HIV is a mutation of simian immunodeficiency virus, a virus found in chimpanzees, and the researchers think it probably entered the human species through infected blood from bush meat.

The report then attributes the spread of the disease to a rapid population expansion, a booming sex trade and unsterilized needles used in health clinics. “The second really interesting aspect is the transport networks that enabled people to move around a huge country,” Oliver Pybus, one of the authors of the study, told the BBC, which reports that over 1 million people were using Kinshasa’s railways by the end of 1940, allowing the disease to spread even more rapidly.

HIV first came to global attention around 1980, and has since affected over 75 million people.

Write to Rishi Iyengar at rishi.iyengar@timeasia.com.