Almost a quarter of new HIV cases are seen in young people, and more than half of them don't know they're infected, says a new report from the U.S. Centers for Disease Control.

According to the report, more than 12,000 new cases occurred in young people aged 13 to 24 in 2010, and close to 60% of them did not know their HIV status.

"That so many young people become infected with HIV each year is a preventable tragedy," wrote CDC Director Dr. Thomas Frieden in the report.

Among the hardest-hit every year are young gay and bisexual men and young African-American men, according to the report. And despite recommendations from the CDC, the American Academy of Pediatrics and the U.S.Preventive Services Task Force that everyone 15 and older receive HIV testing, many teens and young adults still do not get tested.

Researchers found that only about a third of 18-24 year olds and only 13% of high school students have ever been tested.

The CDC also looked at specific risk factors in the young gay and bisexual communities, and found that overall, men having sex with men tended to engage in riskier behavior.

Specifically, they were more likely to have had sex with more than four partners, and more likely to have used injectable drugs - both risk factors for HIV.

In addition, they were more likely to have used drugs or alcohol before having sex, and were less likely to use a condom.

"We can and must achieve a generation that is free from HIV and AIDS," wrote Dr. Kevin Fenton, director of the National Center for HIV/AIDS at the CDC. "It will take a concerted effort at all levels across our nation to empower all young people, especially young gay and bisexual youth, with the tools and resources they need to protect themselves from HIV infection."

The report comes just four days before World AIDS Day on December 1.