A WHO/UNAIDS panel has concluded that treating sexually transmitted diseases can reduce the spread of HIV, both biologically and by causing changes in risky sexual behavior.

Not everybody who is exposed to HIV automatically becomes infected with the deadly virus, according to King Holmes, the director of the Center on AIDS and Sexually Transmitted Diseases (STDs) at the University of Washington.

"We know that the amount of HIV that a person is exposed to, we call that the innoculum size, is the major determinant, certainly a major determinant of the risk of someone who exposed to HIV actually acquiring the infection," said Mr. Holmes.  "And we know that treating an STD for people who have HIV will decrease the amount of HIV that is being shed from the genital tract."

In a study conducted in the mid-1990's in Tanzania, researchers found that the rate of HIV transmission fell by 40 percent when sexually transmitted diseases were treated in both HIV-infected and uninfected individuals.  Other findings have been far less dramatic.

But Tim Farley said experts are fairly convinced that treating STDs is a beneficial prevention strategy.  Farley headed a panel of sexually transmitted disease experts that met in Geneva in July to consider the issue. 

He said there's no question that STDs that cause genital lesions, such as syphilis, gonorrhea and herpes, provide an infection gateway for HIV.

Farley says there's also one STD, herpes,  that may actually make HIV disease worse.

"When patients have active herpes infection, we've seen the viral loads increase," he noted.  "And this is associated in other patients with increased disease progression."

Experts says studies are under way to see whether suppressing a herpes infection also helps treat HIV.

Farley and others say treating patients for STDs also provides an opportunity to educate them about HIV prevention.