More than half a century before AIDS even had a name, the HIV virus was already spreading across Africa via the city now known as Kinshasa, researchers say. In the 1920s, the city was Leopoldville, capital of the Belgian Congo, and scientists say a "perfect storm" of conditions caused the pandemic to emerge, the BBC reports. The fast-growing city was full of transient male workers who visited prostitutes, researchers say. Sexually transmitted diseases were rife, and unsterilized needles at health clinics also helped spread the disease, according to a study published in the journal Science. Researchers studied HIV's genetic code as well as historical documents.
The researchers note that strains of HIV have been transmitted from primates to humans at least 13 times, but only one has caused a global epidemic. They believe a key factor in the 1920s spread was the Congo's booming transport network early last century, which saw a million people travel through Kinshasa every year on the railways alone, reports Reuters. Researchers say the disease was at first confined to specific groups of people, but it started to spread into the general population and around the world after 1960, the Guardian reports. It took genetic research to track the pandemic back to its source because the disease hits the immune system, meaning victims died from any of a wide variety of infections, making the spread of HIV before it was identified hard to track through medical records, says the lead researcher, an evolutionary biologist. (HIV has infected 75 million people, but UN officials say real progress is being made and the epidemic could be over by 2030.)