About one out of every five sexually active gay and bisexual men are infected with HIV, and nearly half of them don't know they have the disease, a recent federal study found.

Among more than 8,000 men in 21 U.S. cities, researchers found that 19 percent of all gay men are infected with HIV. Within the group of those infected, 44 percent of the men were unaware they have HIV, the virus that causes AIDS.

Younger men -- especially young, black men -- are the least likely to know if they have HIV. The study revealed that 28 percent of black men were infected and 60 percent were unaware.

The Kaiser Family Foundation, a research group in Washington, D.C., says less than half of 1 percent of Americans have the AIDS virus. But men continue to be infected with the disease at a much higher rate.

"We don't have a generalized epidemic in the United States. We have a concentrated epidemic among certain populations," explained Jennifer Kates, Kaiser's director of global health and HIV policy.

The new study was conducted in 2008. Researchers offered free testing to the men, interviewed them and paid around $25 for their participation.