By Steve Sternberg, USA TODAY

BOSTON — HIV infection rates in the USA that have remained stable for years now appear to be rising, researchers here reported Tuesday.

"We may be seeing a resurgence of HIV infection in the United States," says Ronald Valdiserri of the Centers for Disease Control and Prevention. "We don't want to be alarmist, but now is the time to address this. We don't want to wait two or three years." (Related story: Promising new AIDS drugs on horizon)

The increase appears to be driven partly by the growing population of sexually active people who are living with HIV coupled with a steep rise in risky behavior.

"Several factors seem to contribute to this high-risk behavior, including fading memories of the early epidemic, illicit drug use and treatment optimism," or an exaggerated faith in the effectiveness of HIV treatment, says Harold Jaffe, also of CDC.

Another factor, Jaffe says, is "the use of the Internet to meet potential sex partners."

Of 900,000 people living with HIV in the USA, about one-third don't know they're infected. Experts have estimated that another 40,000 people become infected each year. But several studies reported at the 10th Conference on Retroviruses and Opportunistic Infections hint that the number could be rising.

Studies show:

New AIDS cases rose by 1% to 41,311 from 2000 to 2001.

From 1999 to 2001, the number of HIV diagnoses grew by 8% in 25 states that report them to the federal government.

Valdiserri cautioned that the statistical snapshot might not be representative because it does not include New York, California and other states that account for three-quarters of AIDS cases nationwide. "We hope to have a more accurate estimate in 2004," Valdiserri said, with reports from all 50 states.

HIV diagnoses among heterosexuals in these states have increased by 10% from 1999 to 2001. The infection rate among homosexual men rose by 14% during that period.

Syphilis rates rose slightly in 2001 for the first time since 1990, only among men, driven by outbreaks in Seattle, San Francisco, Los Angeles, Chicago, Houston, Miami, New York City and Washington, D.C. "A predominant feature of these syphilis outbreaks is that many of these men are HIV-positive," Valdiserri says.

A study of nearly 3,000 men who surf chat rooms on gay Web sites found that 84% met sex partners online and 64% of them had high-risk sex. Eight percent of those with HIV had HIV-negative sex partners.

"The signs are unmistakable," says Cornelius Baker, executive director of the Whitman Walker Clinic, which tracks trends and provides AIDS services in Washington, D.C. "There's the potential for a whole new wave of HIV infections, especially among the young.

"They weren't around when we did education the first time," Baker says. "They haven't experienced as much death and illness. They've become complacent about the very real risks of HIV.

"Until we have a vaccine, until we have a cure, we have to be vigilant. This is a very smart and dangerous virus. We can't let down our guard one tiny bit, or it will prevail."