People who know they have HIV are less likely to transmit the virus than are people who are infected but don't know it.
Taken before AIDS develops, HIV drugs prolong life and health.
People being treated for their HIV infection are less likely to transmit the virus to others.
Most HIV infections come from people who don't know they carry the virus.
At least 200,000 Americans have HIV and don't know it. Many only find out when they develop AIDS.
"Virtually all AIDS cases are preventable, either by preventing infection or by early treatment," Frieden said. "However, 37,000 Americans were diagnosed with AIDS in 2008."
In 2006, the CDC recommended that all Americans get routine HIV tests. Those at high risk of infection should get tested at least once a year, if not more often.
"Forty-four percent of men found to be infected with HIV did not know they were infected, yet half were tested in the past year," Jonathan Mermin, MD, MPH, director of the CDC's HIV/AIDS prevention program, said at the news conference. "This confirms the importance of more frequent testing for those at high risk."