There's some good news from the AIDS front: Fewer needle drug users are testing positive for HIV.

U.S. health officials said Thursday the rate has dropped by half since the 1990s. The decline may be related to a growth in needle exchange programs.

The Centers for Disease Control and Prevention released the study. More than 10,000 drug users in 20 metropolitan areas were surveyed and tested for HIV in 2009. About 1 in 10 tested positive for the virus, compared to roughly 1 in 5 in the 1990s.

The study also found that only about half of infected drug users know they carry the AIDS virus—the first time a CDC study has come up with such a statistic.

Addicts who inject heroin, cocaine or other drugs have been one of the groups hit hardest by the AIDS epidemic. Injection drug users account for about 17 percent of the 1.2 million Americans living with HIV. The CDC recommends drug users get tested at least once a year.

HIV spreads among drug users mainly through unprotected sex and the sharing of needles. Needle exchange programs provide clean needles to addicts, and the number of such programs has grown from nearly 70 in the mid-1990s to more than 180 in 2008.

It's been more than a decade since the CDC last did a large-scale analysis of how common HIV is in injection drug users, said Cyprian Wejnert, the study's lead author.

There was some bad news in the study, too. The proportion of injection drug users tested annually for HIV has dropped from 66 percent in 2006 to 49 percent in 2009. CDC officials say they aren't sure why testing has declined.