Anal sex could be a major cause of the HIV/Aids epidemic in Africa, according to new research.
Millions in Africa are HIV-positive
A study published in The Journal of Sexually Transmitted Diseases and Aids claims awareness of the risks posed by anal sex is ignored in many of the continent's health campaigns.
However, HIV experts reject the findings, maintaining that vaginal sex is the main route of transmission for the Aids virus.
The study by researchers at the University of Tuebingen in Germany proposes that anal sex, both heterosexual and homosexual, is the second biggest cause of HIV transmission in Africa, following transmission in medical settings, such as through dirty needles.
The researchers believe vaginal transmission of HIV is a distant third.
They say there is a presumption that anal sex simply does not happen in Africa and many educational programmes have therefore neglected to cover what they call an "embarrassing topic".
The report's author Dr Stuart Brody, of the Institute of Medical Psychology and Behavioural Neurobiology, said people in Africa needed to be told that anal sex posed a greater risk of transmitting the Aids virus.
"It's quite clear that people in Africa, like people in other parts of the world do engage in anal intercourse and it is reasonably wide spread and therefore has serious health implications," he said.
"The combination of serious illegality and taboo makes it something that it is not likely to generate correct answers when researchers ask and is not likely to be the subject of public health interventions."
Dr Brody admitted some figures from the research, such as interviews with street children in Tanzania, were not representative of the whole population.
But he said the research showed that Africans were less different in sexual behaviour than had been presumed.
But BBC health reporter Ania Lichtarowicz says many doctors dispute the findings, and say the risks of anal sex are generally understood in Africa.
They say that some of the research included in this latest study looked at specific groups of people - for instance prostitutes or street boys - which were not representative of the entire continent.
Vaginal sex, they emphasise, remains the principal route of HIV transmission in Africa.
According to UNAids, an estimated 30m people in sub-Saharan Africa have HIV - 58% of these are women.
Most experts believe that sexual contact is responsible for 90% of HIV transmissions in Africa.