Just over half of new infections with the AIDS virus in the United States are in blacks, U.S. researchers reported on Monday.

A study of detailed data from 33 states shows that of 156,000 new cases of HIV infection between 2001 and 2004, 51 percent were in non-Hispanic blacks - although blacks made up just 13 percent of the population in those states.

The good news is that the rate of new infections declined in black women and in many black men - with the exception of men having sex with other men, the team at the U.S. Centers for Disease Control and Prevention reported.

“Despite reductions in diagnoses among African American men and women, HIV continues to pose a major health threat,” the CDC said in a statement.

“Despite possible signs of success, HIV continues to exact a disproportionate and devastating toll on African Americans.”

The CDC’s Tonji Durant and colleagues found that the rate of HIV diagnosis fell by 6.8 percent annually among black women and 4.4 percent annually among black men between 2001 and 2004.

But blacks still had a substantially higher infection rate than other ethnic groups, the CDC team told the Conference on Retroviruses and Opportunistic Infections underway in Denver.

The HIV diagnosis rate even fell by 9.7 percent every year on average among black male users of injected drugs, the CDC study found.

The human immunodeficiency virus that causes AIDS is spread by sex - homosexual and heterosexual - by sharing needles with someone who is infected, or via blood transfusions. Infected mothers can pass the virus to their babies.

The virus infects close to 40 million people worldwide and more than a million people in the United States.

The CDC estimates that 40,000 Americans become infected with HIV each year.

A second study presented at the same conference found that 32 percent of black men in Baltimore who had sex with other men, which includes homosexual and bisexual activity, were infected with HIV.