Approximately 35 million people are living with HIV and it's "almost certain" their cases can all track back to the original carrier who lived almost 100 years ago.

Researchers from Oxford University and the University of Leuven have determined "with a high degree of certainty" that the first HIV patient came from the African city of Kinshasa, the current capital of the Democratic Republic of Congo, around 1920. The virus spread rapidly through the sub-Saharan region where it's still most prevalent today.

"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," said professor Oliver Pybus of Oxford University of Zoology, senior author of the study.

The international team of researchers will publish the report in this week's issue of Science.

Researchers came to their conclusion by tracing the genetic history of the HIV-1 group M pandemic, the event that started the deadly spread of the virus across Africa and the rest of the world. Urban growth, extensive railway systems and changes to the sex trade between the 1920s and 1950s provided that "perfect storm" for the virus to eventually infect nearly 75 million people.

"We think it is likely that the social changes around the independence [of the Democratic Republic of the Congo] in 1960 saw the virus 'break out' from small groups of infected people to infect the wider population and eventually the world," said Dr. Nuno Faria of Oxford University's Department of Zoology, first author of the paper.

In addition to transportation and the sex trade, new public health initiatives also may have helped spread the disease through the unsafe use of needles.

The first humans contracted the virus from primates and apes, "probably through the hunting or handling of bush meat," according to Pybus.

Sub-Saharan African accounts for 24.7 million or 71 percent of all the active HIV cases in the world, followed by Asia and the Pacific with 4.8 million or 14 percent of cases, according to The Henry J. Kaiser Family Foundation. The first cases were reported in 1981, but efforts to combat HIV didn't occur until the mid-'80s.

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