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Federal health officials are reporting a sharp increase in unprotected sex among gay American men in the past decade, a development that makes it harder to fight the AIDS epidemic.

The same trend has recently been documented among gay men in Canada, Britain, the Netherlands, France and Australia, heightening concerns among public-health officials worldwide.

According to the Centers for Disease Control and Prevention (CDC), the number of men who told federal health investigators they had had unprotected anal sex in the last year rose nearly 20 percent from 2005 to 2011. In the 2011 survey, unprotected sex was more than twice as common among men who said they did not know whether they were infected with HIV.

Being tested even once for HIV is associated with men taking fewer risks, whether the test is positive or negative, health experts say. But the most recent survey found that one-third of the men interviewed had not been tested in the past year.

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The findings are worrying because “unprotected anal intercourse is in a league of its own as far as risk is concerned,” Dr. Thomas Frieden, director of the disease centers, said Wednesday as the figures were released.

The data, published in the agency’s Morbidity and Mortality Weekly Report, do not explain why unprotected sex has risen so rapidly, but a leading hypothesis, Frieden said, is that more men are “sero-sorting” — that is, those who are uninfected (“HIV seronegative” on lab reports) try to sleep only with other men who are uninfected, or who hope they are, or who merely promise they are.

“The problem with sero-sorting is that it’s really easy to get it wrong,” Frieden said. “When one-third of men aren’t even tested in the last year and a tenth of those who thought they were negative were actually positive, you don’t want to risk your life on a guess.”

Other hypotheses, said Frieden and Jonathan Mermin, the CDC’s director of HIV prevention, are that many young men have never known anyone dying of AIDS and so do not fear it, or that they believe they can easily stay on antiretroviral drugs for life.

“Young guys are less worried,” said Alex Carballo-Diéguez, a researcher at the HIV Center of the New York State Psychiatric Institute and Columbia University who has studied gay men’s behavior since the 1980s. “HIV has become a chronic disease, and everyone knows some behaviors are bad for you, like smoking and trans fats. But in the moment of excitement, they’re going to do what they enjoy.”

Perry Halkitis, a researcher at New York University, attributes the rise of unprotected sex to two factors.

First, recent studies have shown that people who take their antiretroviral drugs daily are very unlikely to transmit disease, so uninfected men think it is relatively safe to sleep with them. A problem with that line of thinking, however, is that not all men on the drugs take them every day. Second, he said, the collapse of the economy in the past six years has put many young men out of work, “and we see higher-risk behavior when people have more risk in their lives.”

Since 2005, the CDC has been conducting the National HIV Behavioral Surveillance System survey every three years in 20 cities, mostly in gay bars, but also in parks and on streets in gay neighborhoods. The agency concedes it probably undercounts some high-risk groups, including prisoners, teenagers too young or poor to go to bars and men who keep their homosexuality secret.

Surveys have consistently found infection rates highest among young black gay men, even though they were more likely than their white counterparts to use condoms. The chief reason for that, experts say, is that most people have sex within their own ethnic groups and older black gay men have very high rates of undiagnosed and untreated HIV.

The number of new HIV infections in the U.S. has been stuck at roughly 50,000 a year for many years, although public-health officials are trying to bring it down. Among the factors working against them: The U.S. population is growing, and infected men are living longer and staying sexually active thanks to antiretroviral drugs.

The goal of the national AIDS strategy put in place by the Obama administration in 2010 is to have new infections down to roughly 38,000 a year by 2015.

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