A new scientific study has pinpointed the origins of the HIV/AIDS epidemic. In a paper published Thursday in Science, University of Oxford evolutionary biologist and infectious-disease specialist Oliver Pybus used HIV genomes from 814 people in central Africa to reconstruct a viral evolution “family tree.” That method traces the epidemic back to the 1920s in the city of Kinshasa in the modern-day Democratic Republic of the Congo. Many viruses never grow larger than an outbreak, but HIV became a global pandemic thanks to Congo’s growing railway network. Built by the French during the colonial era, the rails transported millions of male wage laborers. Prostitutes followed in their wake. As a result, the Group M strain of HIV (responsible for 90 percent of infections) took off in the 1960s, as workers traveled from Kinshasa to other cities. The virus then spread across continents when many of the workers from Haiti returned home in the 1960s. AIDS was not diagnosed until 1981, when the Centers for Disease Control and Prevention reported a unique type of pneumonia in five gay men in Los Angeles. HIV was discovered in 1984. Since then, more than 36 million people have died.