"In Africa, HIV prevalence is high in young women and that's the picture we have of what's driving the epidemic," Professor Salim Abdool Karim, director of HIV/Aids research institution Caprisa, told the M2010 Microbicides conference this week.
"What's been forgotten is the hidden side of the epidemic, since same-sex relationships are criminalised in 37 out of 54 countries in sub-Saharan Africa.
"In reality, HIV is really common among gay men throughout Africa. We simply don't talk about it."
The scientists at the conference, in Pittsburgh in the US, said a rectal microbicide could help reduce infections.
Studies show that men who have sex with men are at high risk of HIV infection - particularly in Africa, where safe access to prevention tools and services is restricted.
Up to 3% of men in South Africa are thought to have had homosexual intercourse. In Tanzania the estimate is 2% to 3%, and in Kenya up to 0.9%.
About a third of these men report they are married or in stable heterosexual relationships. The overlapping of sexual networks allows the virus to thrive.
Viral fingerprinting (genotype data) has found that the strains of HIV circulating across gay and heterosexual networks match one another.
South Africa has one of the worst HIV prevalence rates in the world, and prevalence among men who have sex with men is similar to that of the general population, estimated at about 13%.
HIV prevention efforts aimed at men who sleep with men are needed - about 20% of new infections are among this group - yet health activists are hamstrung by widespread political and cultural hostility to same-sex relationships.
Michel Kazatchkine, executive director of the Global Fund to fight Aids, TB and Malaria, said in response to the recent imprisonment in Malawi of two men for being gay: "The criminalisation of individuals based on their sexual orientation is not just a human rights issue - it also ... drives sexual behaviour underground and creates an environment where HIV can more easily spread.
"This ultimately affects the broader population, in addition to the devastating impact it has on men who have sex with men."
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