Some time around 1920, a person carried a virus down the Sangha River, from Cameroon toward the capital of the Democratic Republic of the Congo. The virus was a strain of HIV, and the city—then called Léopoldville and, now, Kinshasa—gave it the perfect soup of conditions to ignite the AIDS epidemic. Since then, HIV has infected nearly 75 million people worldwide.
A new study, published in Science, looks at how HIV, an infection that had previously affected people only in the immediate region of its origins could bloom into one that crossed international boarders.
Researchers already knew that chimpanzees in southern Cameroon harbor viruses most closely resembling HIV-1, group M, the strain that went global. By comparing the genetic changes between different strains, researchers had figured out that HIV-1’s lineage made the leap from chimp to human some time in the early 1900s. In fact, HIV likely jumped several times to people handling bushmeat, but only one strain created the pandemic we grapple with today.
The researchers combed through the genetics of hundreds of tissue samples from people infected with HIV from the last 50 years. By creating a kind of virus family tree, they traced back and discovered a common ancestor from about 1920 in Kinshasa.
Contrary to some theories, the new study suggests that there wasn’t any thing special about that group M strain. “Perhaps the [new study’s] most contentious suggestion is that the spread of the M-group viruses had more to do with the conditions being right than it had to do with these viruses being better adapted for transmission and growth in humans,” scientist Jonathan Ball of the University of Nottingham told the BBC.
At that time, Kinshasa’s population was booming. The Congo river connected the growing city to Kisangani, and rail lines carried hundreds of thousands of passengers to major mining locations Lubumbashi and Katanga. With the influx of largely male laborers came many sex workers. Contaminated needles may have also played a role.
“There were lots of different factors,” lead author Oliver Pybus, an infectious disease researchers from the University of Oxford in the United Kingdom, told Science Magazine. “Basically this one was at the right time and the right place—and it hit the jackpot.”