A new study on the genesis of HIV argues that the virus initially spread from its more localized origins in central Africa out of Kinshasa, the capital of the Democratic Republic of the Congo (DRC), probably during the 1920s, BBC News Health reports. Publishing their findings in the journal Science, researchers genetically reconstructed the history of what is known as “group M” of the HIV pandemic in order to come to this conclusion. The confidence interval suggests that that HIV may have emerged from Kinshasa—then part of the Belgian Congo and called Leopoldville—as early as 1909 or as late as 1930.

This study does not contradict previous research suggesting that HIV made its fateful leap from chimps to humans in neighboring Cameroon, probably through the hunting or handling of bush meat at around the turn of the 20th century. Rather, it considers how one version of HIV out of what was actually at least 13 cross-species jumps later spread widely and led to a cumulative 75 million infections today.

“Until now most studies have taken a piecemeal approach to HIV’s genetic history, looking at particular HIV genomes in particular locations,” Oliver Pybus, PhD, of Oxford University’s department of zoology and a senior author of the paper, said in a release. “For the first time we have analyzed all the available evidence using the latest phylogeographic techniques, which enable us to statistically estimate where a virus comes from. This means we can say with a high degree of certainty where and when the HIV pandemic originated. It seems a combination of factors in Kinshasa in the early 20th century created a ‘perfect storm’ for the emergence of HIV, leading to a generalized epidemic with unstoppable momentum that unrolled across sub-Saharan Africa.”

These factors included the fact that Kinshasa was a major railway hub in the early 20th century, that male workers who came into the city in droves began engaging in a thriving sex trade, and that the local population was growing quickly. Unsterilized needles used in medical facilities may have also spread HIV.

The virus apparently traveled along railways and waterways between about 1920 and 1960, eventually reaching the DRC cities of Pointe-Noire to the west of Kinshasa, Bwamanda to the north, Kisangani to the northeast, and Lubumbashi to the southeast. These are vast distances to travel, considering that DRC is the size of Western Europe.

DRC’s independence from Belgium in 1960 may have catalyzed HIV’s spread around the world.

To read the BBC report, click here.

To read the press release, click here.

To read the study abstract, click here.