A US study estimates that almost half of HIV-positive gay and bisexual men are unaware they are infected.

The Centers for Disease Control and Prevention (CDC) study also suggested that one in five of gay men in cities most affected by HIV have the virus.

The research studied more than 8,000 gay and bisexual men in the 21 cities which have the highest HIV rates. They were given HIV tests by CDC researchers.

In Baltimore, 38 per cent of the men tested were found to be HIV-positive, while just six per cent in Atlanta tested positive.

The highest number of infections occurred in black gay and bisexual men. This group were less likely to be aware of their infection with 59 per cent unaware, compared to 46 per cent of Hispanic men and 26 per cent of white men.

Men under the age of 30 were significantly less likely to be aware of having HIV. Sixty-three per cent of HIV-positive gay and bisexual men between the ages of 18 and 30 did not know they had the virus, compared to 37 per cent of HIV-positive gay and bisexual men over the age of 30.

Dr Jonathan Mermin, director of CDC’s Division of HIV/AIDS Prevention, told Reuters: “We can’t allow HIV to continue its devastating toll among gay and bisexual men, and in particular, among young black men.

“We need to reinvigorate our response to preventing HIV among gay and bisexual men.”

“The severity of the impact of HIV in the gay community is nothing new,” said Carl Schmid of the non-profit AIDS Institute.

“What has been missing is an appropriate response by our government, at the federal, state, and local levels, and the gay community itself.”

The research was published ahead of National Gay Men’s HIV/AIDS Awareness Day, which is on Monday September 27th.

According to CDC, gay and bisexual men represent approximately two per cent of the US population, yet are the population most severely affected by HIV and are the only risk group in which new HIV infections have been increasing steadily since the early 1990s.