New HIV infections occur more among gay and bisexual men. Blacks are most at risk, a study by the Centers for Disease Control and Prevention (CDC) says.
The data also show that whites and blacks tend to be infected at different times in their lives.
Most new infections of white gay and bisexual men occur when the men are in their 30s and 40s, the study found, while black gay and bisexual men are more likely to be infected in their teens and 20s.
The CDC reported last month that the study found that the virus was spreading faster in the United States than had been thought. In 2006, the study found, 56,300 people were newly infected with H.I.V. 40 percent more than the agency's previous estimate of roughly 40,000 new cases a year. The study was performed using new technology that allowed researchers to distinguish between new and older infections.
The details of the agency's demographic analysis were released on Thursday in the hope that knowledge of the age, race and other characteristics of the newly infected would better direct prevention efforts.
"The data really confirm what we had suspected and known before, that it has disproportionate impact on gay and bisexual men and on blacks and Latinos." said Dr. Dr. Kevin Fenton of the CDC.
Black people, who make up about 12 percent of the population, accounted for more than 45 percent of the new infections, the study found, and the disparity was particularly acute among women.
Black women are nearly 15 times as likely to be infected with H.I.V. as white women. Hispanic women are four times as likely to be infected as white women. Black men have six times the H.I.V. incidence rate of white men and nearly three times that of Hispanic men.
Among those newly infected with the virus, black men were no more likely to be drug users or to engage in risky sex than were white men, according to the study. More research is needed to explain why young black men are at such greater risk for contracting the disease, but there are several hints from other studies, researchers said.
The fact that proportionally more blacks than whites are already infected would tend to produce higher transmission rates among blacks, said Dr. Richard Wolitski, acting director of the center's division for H.I.V. and AIDS prevention. Young black men are much more likely to have been incarcerated. Infection rates among former convicts are high, largely because of behaviors outside of prison, studies show.
Dr. Wolitski said young black gay and bisexual men also tended to have partners who were older than their white counterparts and thus were more likely to have already been infected.
Girls and women make up 27 percent of those newly infected with the virus, and 80 percent of them contracted H.I.V. because of high-risk heterosexual contact. Among newly infected males, 81 percent of white men and 63 percent of black men were gay or bisexual.
In one of the most dismal statistics provided by the centers, researchers said that 80 percent of gay and bisexual men in 15 cities had not been reached by intensive H.I.V. prevention efforts that have proven effective. Agency officials said that more must be done, including expanded H.I.V. screening programs and better directing of prevention efforts at those most at risk.