A new scientific research in the genetic history of HIV has revealed that the pandemic behind the deaths of 36 million people had most likely originated in the Democratic Republic of Congo in the 1920s.
The virus causing AIDS was assisted by train transport and the sex trade, facilitating its spread from the city of Kinsasha to the rest of the continent and eventually around the world, infecting some 75 million people.
"For the first time we have analysed all the available evidence using the latest phylogeographic techniques, which enable us to statistically estimate where a virus comes from," senior author Oliver Pybus, of Oxford University's Department of Zoology, said according to the AFP news agency.
While various strains of the HIV have jumped from primates and apes to humans at least 13 times, only one such transmission event has led to a human pandemic, as it was aided by "a 'perfect storm' of factors, including urban growth, strong railway links during Belgian colonial rule, and changes to the sex trade.
These factors combined saw the emergence of HIV from Kinshasa and its spread across the globe" between the 1920s and 1950s, said the study in the journal Science.
"This means we can say with a high degree of certainty where and when the HIV pandemic originated."
The use of trains as transport helped bring the virus from isolated pockets of people into the larger city, which was Kinshasa, among the best connected of all central African cities.
"Data from colonial archives tells us that by the end of 1940s over one million people were travelling through Kinshasa on the railways each year," said Nuno Faria of Oxford University's Department of Zoology, first author of the paper.
Social changes at the time included sex workers who took on a large number of clients, coupled with "public health initiatives against other diseases that led to the unsafe use of needles (which) may have contributed to turning HIV into a full-blown epidemic," the study said.
Campaigns to treat people with sexually transmitted diseases may have been carried out using needles that were not sterile.
HIV was first identified in 1981, and the AIDS epidemic ballooned for more than a decade until antiretroviral drugs were created. These long-term regimens have transformed HIV from a fatal disease into a chronic condition for many of those infected.
Researchers said further study is needed to understand the different social factors that enabled the virus to spread the way it did.