In 2005, over half of new HIV infections diagnosed in the US were among gay men, and up to one in five gay men living in cities is thought to be HIV positive.

Yet two large population surveys showed that most gay men had similar numbers of unprotected sexual partners per year as straight men and women.

US researchers applied a series of carefully calculated equations in different scenarios to study the rate at which HIV infection has spread among gay men and straight men and women.

They used figures taken from two national surveys to estimate how many sex partners gay men and straight men and women have, and what proportion of gay men have insertive or receptive anal sex, or both.

They then set these figures against accepted estimates of how easily HIV is transmitted by vaginal and anal sex to calculate the size of the HIV epidemic in gay men and straight men and women.

The results showed that for the straight US population to experience an epidemic of HIV infection as great as that of gay men, they would need to average almost five unprotected sexual partners every year.

This is a rate almost three times that of gay men.

But to end the HIV epidemic, gay men would need to have rates of unprotected sex several times lower than those currently evident among the straight population. This is because transmission rates are higher for anal sex than they are for vaginal sex, say the authors.

But "role versatility," whereby people adopt both "insertive" and "receptive roles," also plays a part, they add.

A gay man can be easily infected through unprotected receptive sex, and then infect someone else through insertive sex.

Gay men are therefore far more susceptible to the spread of the virus through the population, even with the same numbers of unprotected sexual partners.

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