Women in South Africa might be biologically predisposed to contracting HIV, explaining in part why local women are among the world's most susceptible.
The US Centres for Disease Control estimates that, on average, a woman would have to have sex 1,000 times with an HIV-positive male to get the virus. But this estimate does not hold in South Africa, particularly in rural KwaZulu-Natal, where more than half of pregnant women will be HIV-positive by the time they are 25 or older.
Deputy director of the Wits Centre for Reproductive Health and HIV Francois Venter said: "I'm deeply sceptical that sexual behaviour alone drives the epidemic in our area.
"What kind of sex are we Southern Africans having that confers a several thousandfold risk on a young woman in KwaZulu-Natal versus her contemporary in London or Delhi?"
Research published on Friday by scientists at the Centre for the Aids Programme of Research in SA, show that KwaZulu-Natal women who acquired HIV had more immune cells in their vaginas than those who did not.
Salim Abdool Karim, the centre's director, said: "This research provides evidence that the high HIV risk in women is not simply because of behaviour but has a biological basis."
This means that "in trying to reduce HIV in young women we might have been barking up the wrong tree by focusing only on trying to change their behaviour".
The centre's scientists have monitored thousands of KwaZulu-Natal women for years, taking vaginal cell samples every six months for a range of HIV-prevention trials.
Their latest study – published in the Clinical Infectious Diseases journal – reveals that 58 KwaZulu-Natal women who contracted HIV had vaginas more "friendly" to the HI virus than women from the same community who had the same number of sexual partners and sexual encounters.
After the women became HIV-positive researchers retrieved frozen genital cell samples taken from them before they became infected.
These samples were compared to those from HIV-negative women in the same community.
Those who became HIV-positive had higher rates of genital inflammation, caused when the body's immune system mobilises to fight an infection or other form of attack on the body. Inflammation causes large numbers of CD-4 (immune) cells to rush to an area to defeat the threat. These immune cells are the ones HIV infects.
Karim said: "Something is causing an inflammatory response in the vagina, leading to an inflow of CD-4+ cells and thereby increasing the risk of HIV infection.
"We have not yet found the cause of the inflammation."
Sexually transmitted infections could not account for most of the inflammation found in the study.
University of Cape Town researchers Jo-Ann Passmore and Lindi Masson, who took part in the research, said similar studies by them in other parts of South Africa showed the same results: inflammation in the vagina leading to a higher risk of contracting HIV.
Masson is working on developing a test to detect inflammation of the vagina in the absence of symptoms.
Venter, who was not involved in the study, said: "This is important research. It allows us to start thinking about ways to intervene. If genital inflammation is the reason, understanding why [it arises] and finding ways to decrease it seems a good HIV-prevention step."
In follow-up studies, researchers are investigating whether hormone changes, hygiene or changes in the bacteria populating the vagina could be causing inflammation.