A new study suggests that HIV jumped from apes to humans around the turn of the 20th century, which coincides with the development of colonial cities in sub-Saharan Africa.
HIV was and remains a "relatively poorly transmitted" virus, he said, so the key to the success of the virus was possibly the development of cities such as Leopoldville in the early 1900s.
The large numbers of people living in close proximity would have allowed more opportunity for new infections.
"I think the picture that has emerged here, is that changes the human population experienced may have opened to the door to the spread of HIV," he said.