(AP) -- One in five sexually active gay and bisexual men has the AIDS virus, and nearly half of those don't know they are infected, a federal study of 21 U.S. cities shows.
Experts said the findings are similar to earlier research, but the study released Thursday is the largest to look at gay and bisexual U.S. men at high risk for HIV. More than 8,000 men were tested and interviewed, and 44 percent of those who had the virus didn't know they had it.
Overall, less than half of 1 percent of Americans have the AIDS virus, according to a calculation by the Kaiser Family Foundation, a research and policy organization in Washington, D.C.
But gay and bisexual men continue to be infected at much higher rates, said Jennifer Kates, Kaiser's director of global health and HIV policy.
"We don't have a generalized epidemic in the United States. We have a concentrated epidemic among certain populations," she said.
That's why a new national AIDS strategy, unveiled by the White House in July, is emphasizing more of a government focus on men who have sex with men and others at the highest risk of getting infected, Kates said.
The Centers for Disease Control and Prevention recommends HIV testing at least once a year for all men who have sex with men and are sexually active, but research indicates more than half don't get tested.
An earlier study in just five cities in 2004-05 found similar results.
The new study, conducted in 2008, included 16 additional cities. Researchers offered free testing to the men, interviewed them and paid around $25 for their participation.
Black men were more likely to have HIV, with 28 percent reportedly infected, compared to 18 percent of Hispanic men and 16 percent of white men.
Black men were also least likely to know they were infected - about 60 percent didn't know they had HIV - compared 46 percent of Hispanic men and 26 percent of whites.
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More information: CDC report: http://www.cdc.gov/mmwr