A new study into the origins of the AIDS virus suggests one strain of HIV appeared in the early 20th century in the western region of Congo and spread through a swath of Africa over the next several decades without notice by the rest of the world.
The researchers say the findings support, but don't prove, the theory that the virus expanded its reach in Africa due to social factors such as railroad expansion, changing sexual habits and unsafe medical practices.
Read: The causes of HIV
How HIV became a pandemic
The study adds to our understanding of "how a virus that is less transmissible than other pathogens like malaria and the common cold can still become established in the human population and eventually grow into a devastating pandemic," said study co-author Philippe Lemey, from the Rega Institute for Medical Research at Catholic University of Leuven, Belgium.
"The fact that social changes were critical in the rise of the virus suggests that such changes may also be an important factor in combating epidemic spread," Lemey said.
Read: HIV: a global pandemic
The origins of HIV, the sexually transmitted virus that causes AIDS, are still hazy. Scientists believe variations of the virus migrated from primates, possibly monkeys and chimps, to humans in Africa. Then two strains of the virus, known as HIV-1 and HIV-2, developed in people.
The new study, published in the Oct. 3 issue in the journal Science, looks at a form of HIV-1, the prevalent strain in the world today. Scientists previously determined that it existed in heterosexual populations in the first half of the 20th century, but exactly where and when it appeared wasn't clear.
Initial appearance of HIV
Now, the study authors say analysis of virus samples suggests this form of HIV emerged in Kinshasa, the capital of what is now known as the Democratic Republic of the Congo, in the early 1920s. The researchers relied on a "molecular clock" model to determine the timing.
"Its initial spread followed the Democratic Republic of Congo's transport links, in particular its railways, that made Kinshasa one of the best connected of all central African cities," Lemey said. This new ease of movement, coupled with prostitution (a byproduct of development) and use of unsterilized needles by medical workers treating sex workers may have boosted transmission of the virus, he said.
Why these findings matter?
Lemey said the research reveals a "perfect storm" of social changes that may have fostered HIV's spread. This matters now, he said, because "knowledge of the circumstances that facilitated the expansion of the epidemic can assist the development of effective education and prevention programs."
According to Lemey, the virus may have gone undetected because few people were infected at first and because the symptoms caused by the AIDS-related decline of the body's immune system were "definitely not new to Africa." AIDS wasn't formally recognized until 1981 after it had spread across the world. Since then about 75 million people have been infected, according to a United Nations report.
Lemey cautioned that the findings don't offer insight into how a variation of the virus leaped from animals to people, or what might have happened if the virus had been recognized decades ago. There's no way to know if medical officials could have stopped its spread.
Still, the study authors say the findings could be a template for understanding the spread of other blood-transmitted diseases such as hepatitis C.
Currently, researchers around the world are struggling to contain Ebola infections in Africa.
William Schneider, a professor who studies the history of medicine at Indiana University-Purdue University Indianapolis, praised the study for providing "a fascinating and helpful framework" to understand the origins of HIV/AIDS. Schneider is part of a team of historians and anthropologists exploring the roots of the disease in Africa.
However, Schneider said the authors' theories about the spread of the disease aren't convincing because they're based on questionable information from a time when Europeans controlled much of Africa.
"Colonial records are well-known by historians and anthropologists to be biased, selective and above all in need of broader understanding of historical and cultural circumstances to evaluate their usefulness," he said.
For more about HIV/AIDS, visit our HIV/AIDS centre or see the U.S. National Institutes of Health.
Preventing HIV in Africa
HIV prevention drugs raise more questions than answersWhy is HIV still winning?