The research was funded in part by the Wellcome Trust, St. John's College Oxford, the Rhodes Trust, the Royal Society, the National Institutes of Health and the Howard Hughes Medical Institute.
Previous studies have linked all variants of HIV to the chimpanzee subspecies native to the countries to the west of the Democratic Republic of Congo (DRC).
However, genetic diversity of the pandemic strain of HIV, known as HIV-1 group M, is highest in the DRC.
So the DRC is an important place to look for clues about the origins of AIDS and specifically to test the oral polio vaccine/AIDS theory.
"I wanted not just to test the OPV/AIDS theory, but also to collect fundamental data from a crucial area," Worobey said.
Easier said than done.
Although recent technological advances have made it possible to test feces for RNA, the genetic material of SIV and HIV, one still has to collect the specimens.
Despite civil war in the region, Worobey traveled to the Democratic Republic of Congo in 2000 and in 2003 to collect fresh samples from wild chimps in the Kisangani area.
"The fresher, the better," he said. "Basically you want to pick it up the same morning they let it go."
He worked with a team of hunters that track the chimps at night using their calls. The team carefully stored small samples of feces in little screw-capped vials filled with a preservative.
The 2000 trip met with some calamities. Worobey contracted blood poisoning from a rattan palm spike that stabbed his hand. He hiked out of the forest to seek medical attention. His two companions, Jeffrey Joy and the well-known evolutionary biologist W.D. (Bill) Hamilton, came down with malaria. Hamilton later died from complications.