Despite years of great progress in treating AIDS, the number of new infections with the virus that causes it has remained stubbornly around 50,000 a year for a decade in the United States, according to new figures released on Wednesday by federal officials.

The American epidemic is still concentrated primarily in gay men, and is growing rapidly worse among young black gay men.

That realization is causing a rift in the AIDS community. Activists say the persistent H.I.V. infection rate proves that government prevention policy is a flop. Federal officials are on the defensive even as they concede that the epidemic will grow if prevention doesn’t get better, which they know is unlikely while their budgets are being cut.

And some researchers believe it is impossible to completely wipe out even a fatal, incurable disease when it is transmitted through sex and carries so much stigma that people deny having it and avoid being tested for it.