An international team of scientists say a “perfect storm” of population growth, sex and railways allowed HIV to spread.
A feat of viral archaeology was used to find the pandemic’s origin, the team report in the journal Science.
They used archived samples of HIV’s genetic code to trace its source, with evidence pointing to 1920s Kinshasa.
Their report says a roaring sex trade, rapid population growth and unsterilised needles used in health clinics probably spread the virus.
Meanwhile Belgium-backed railways had one million people flowing through the city each year, taking the virus to neighbouring regions.