Washington ? Some 250,000 Americans are HIV positive but unaware of it, according to a Centers for Disease Control and Prevention study released Thursday, and most of them are not in high-risk groups.

“In the past, people associated HIV with drug use and men who have sex with men,” said Bernard Branson, associate director for Laboratory Diagnostics in CDC’s division of HIV/AIDS prevention. “But the epidemic is changing, and there is an increased proportion of cases that have been reported in heterosexual transmissions and among women.”

Branson said efforts to test people who are in high-risk groups for HIV had been successful. To reach the remaining 25 percent of Americans who are HIV positive but don’t suspect it, however, efforts have to be broadened, and quickly, according to Branson.

An HIV-positive person who doesn’t know it is three and a half times more likely to transmit the infection than someone who does, he said.

The CDC study found that testing rates for HIV rose sharply from 1987 to 1997, then less steadily. From 2001 through 2006, the testing rate stalled.

Currently, about 40 percent of Americans aged 18 to 64 say they’ve been tested for HIV at least once.

The CDC study also found that people with high risk factors – gay men, needle drug users, hemophiliacs, prostitutes and people with HIV-positive partners – were more likely to get tested than others.

To alert those who are HIV positive and don’t know it – and those who are at high risk and need frequent testing – the CDC recommended in 2006 that HIV testing be part of routine medical care.

According to Branson, the CDC found that when testing was part of a medical routine, it was “highly” accepted because people didn’t feel singled out.