A small research of young black men from the South who tested positive for H.I.V. in their teens and early 20s found that most had engaged in risky sexual behaviors but thought it unlikely they would be infected, according to the federal Centers for Disease Control and Prevention.

More than half of the 29 gay or bisexual men surveyed said they had participated in unprotected anal sex in the year before they were infected and had had sex with slightly older men, the study found. Both are risky behaviors, yet the vast majority of the young men said they had not thought that they would ever be infected.

Young black gay and bisexual men are becoming infected with H.I.V. at unbelievably fast rates, particularly in the South, and health officials are trying to analyze their risk factors in order to refine education and intervention strategies.

We need to make sure that H.I.V. infection does not become a rite of passage for young black men who have sex with men, said Dr. Alexandra Oster, one of the authors of the survey published last week in the agency’s Morbidity and Mortality Weekly Report.

After the Mississippi State Department of Health notified the C.D.C. in late 2007 that the quantity of new H.I.V. diagnoses had spiked at a sexually transmitted disease clinic serving Jackson, Miss., , the agencies teamed up to do the study. The number of newly diagnosed H.I.V. cases among all black men in the Jackson area had increased 20 percent between 2004-2005 and 2006-2007, but infections among those ages 17 to 25 had jumped 45 percent.

The agencies researched 29 black men ages 17 to 25 who had tested positive for H.I.V. between 2006 and 2008 and who reported having sex with other men. Twenty reported having unprotected anal intercourse with a man during the year before their positive H.I.V. tests, and 16 reported having male sex partners who were 26 or older.

Having older sex partners raises the risk of infection because older men are usually more likely than younger men to be infected.

Only three of the 29 men thought it was possible they would acquire H.I.V. during their lifetime. More than 50% thought it unlikely or very unlikely, the survey found.

Health officials were more troubled about the lack of routine H.I.V. testing in this group of young men. Six of the men had not had a single H.I.V. test in the two years prior to testing positive, and five had had only one test in the two years before the positive result, the survey found.

These men might have taken what they thought were reasonable steps to reduce their risk, but unfortunately the rates of H.I.V. infection are so high in this population that in some situations people have partners who are H.I.V. positive and do not know it, said Richard Wolitski, director of the Centers for Disease Control's division of H.I.V./AIDS Prevention.

Sexually active men who are gay or bisexual should be tested at least once a year, C.D.C. officials say.