"It is the question we are asked most often – why is the situation so bad in sub-Saharan Africa? It is a combination of factors – more commercial sex workers, more ulcerative sexually transmitted diseases, a young population and concurrent sexual partnerships."

"Sexual behaviour is obviously important but it doesn't seem to explain [all] the differences between populations. Even if the total number of sexual partners [in sub-Saharan Africa] is no greater than in the UK, there seems to be a higher frequency of overlapping sexual partnerships creating sexual networks that, from an epidemiological point of view, are more efficient at spreading infection."

Low rates of circumcision, which is protective, and high rates of genital herpes, which causes ulcers on the genitals through which the virus can enter the body, also contributed to Africa's heterosexual epidemic.

But the factors driving HIV were still not fully understood, he said.