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ATLANTA — The proportion of men in the United States having unprotected sex with other men increased 20 percent from 2005 to 2011, according to a federal study released on Wednesday, raising new concerns over the spread of the HIV virus, which causes AIDS.



Fifty-seven percent of men who have sex with other men reported having unprotected anal sex at least once in the last 12 months, up from 48 percent in 2005, according to the Centers for Disease Control and Prevention.

The study of more than 9,000 men included interviews and HIV testing.

"We are concerned about the increase," the study's lead author, Gabriella Paz-Bailey, told Reuters.

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An estimated 50,000 people in the United States are infected with HIV each year, a number that has remained steady in recent years, Paz-Bailey said.

Men having sex with other men account for 65 percent of new infections even though they represent only 2 percent of the total population, she added.

The reason for the increase in unprotected anal sex is unclear, but could be because men are increasingly having sex with partners who have the same HIV status as their own in the belief that this lowers their risk of contracting the virus, the study said.

However, the CDC warns that this could be a dangerous strategy.

Men with HIV "might not know or disclose that they are infected and men's assumptions about the HIV status of their partners can be wrong," the agency said.

Highlighting the need for increased testing, the study found that men who are not aware of their HIV status are much more likely to have unprotected anal sex with other men than those who have been tested for the virus.

"We think that testing is one our greatest weapons," Paz-Bailey said. "We know that when men are tested and know their status they are likely to change their behavior."

CDC recommends annual HIV testing for those at high risk of contracting the virus, but in the study only 67 percent of men having sex with other men said they had been tested within the past year.