A wall of silence, repression and discrimination are amplifying dangers for men who have sex with men in sub-Saharan Africa, they said in a paper published online on Monday.
Researchers from the University of Oxford looked at published studies for human immunodeficiency virus (HIV) prevalence from 2003 to 2009.
Prevalence among gays in some parts of West Africa is 10 times that for the general male population, they found.
The difference varies a lot across Africa, but in most of the countries studied, the rates among homosexuals were substantially higher than among heterosexuals.
Political, religious and social hostility towards homosexuality is entrenched in many countries, and this breeds isolation, harassment and prejudice, enabling risky sex practices to multiply, the paper said.
"Unprotected anal sex is commonplace, knowledge and access to inappropriate risk prevention measure are inadequate and... in some contexts, many MSM [men who have sex with men] engage in transactional sex," it said.
The paper said secrecy was so entrenched that data about gay sex behaviour in Africa was often sketchy or absent.
"There's surprisingly little known," said lead investigator Adrian Smith.
"(...) [W]hat little evidence we do have suggests that MSM are a vulnerable group that exists across sub-Saharan Africa."
The review stressed that the risks were not limited to gays, as many MSM also have sex with women.
"In the early 1980s, silence equals death became a rallying cry" for gays in the United States, it said.
"Nearly three decades later in sub-Saharan African the silence remains, driven by cultural, religious, and political unwillingness to accept MSM as equal members of society."
Around 33 million people have HIV, according to figures issued in 2008 by the UN agency UNAIDS. Two-thirds of them live south of the Sahara.