According to the CDC's Division of HIV/AIDS Prevention, HIV diagnoses are down by one-third overall, but have more than doubled for young men who have sex with men. "It's been more than 30 years since the first cases were reported," says the study's co-author, Amy Lansky, "It's harder to maintain that sense of urgency."

The report, published in The Journal of the American Medical Association, examined HIV diagnoses among Americans aged 13 and older from 2002 to 2011 and determined the annual HIV rate fell from 24 out of every 100,000 people to 16—a 33% decline.

Among the groups experiencing declines, the report indicates:

Among women, diagnosis rates dropped by about half, and among men by more than one-quarter. For blacks and Hispanics, the rates of diagnosis declined 37 percent and 41 percent, respectively.

The report estimates that HIV infections due to injection drug use fell by roughly 70 percent and from heterosexual contact by more than one-third for men and women.

Distribution of sterile needles, increased HIV testing and drug-treatment programs could explain some of the downward trend, Lansky said.

Gay and bi men aged 13-24, however, saw their diagnosis rate rise from about 3,000 to about 7,000.

Some medical experts are urging for the use of pre-exposure therapies like Truvada for all HIV-negative men who have sex with men. "The use of antivirals to prevent HIV infection is fraught with many challenges," says Dr. David Margolis, an AIDS specialist at the University of North Carolina at Chapel Hill, "but if there is a more than doubling of new infections in one demographic, perhaps something needs to be done."