In what may be the clearest example yet of an Internet community having an impact on a real-world community, a gay and lesbian Web site has teamed with a public health department to track down people infected with syphilis and give them treatment information.

A campaign of e-mail, Web postings and instant messaging by Web site PlanetOut is being credited by San Francisco's public health department for doubling the number of men coming into the city's clinics for syphilis testing.

When six San Francisco men and two men from neighboring cities were questioned by doctors about how they might have contracted the disease--the incidence of which is at an all-time low--it turned out that each had met recent sex partners on America Online chat rooms called SF M4M, for "San Francisco Men for Men." Several of the men also have HIV, the virus that causes AIDS.

"We've been trying to identify sexual networks," where people meet for anonymous sex, said Jeffrey D. Klausner, director of sexually transmitted disease prevention and control services at the San Francisco department of public health. But usually the networks he's been watching meet in parks, parties or bars, not online. "This has been our first effort to do prevention and control in cyberspace," said Klausner.

And Klausner discovered that on the Internet, it's much easier to find someone. Even though many of the men did not know each other's names, they did have another clue: an AOL screen name.

Klausner's first thought was to go to AOL and ask for contact information. But AOL has strict rules about protecting its subscribers' privacy, and only reveals information as part of a legal action.

So Klausner decided to use his traditional methods, simply asking for the screen names from those infected.

At the same time, AOL put Klausner in touch with PlanetOut, a gay and lesbian Web site based in San Francisco that is partly owned by AOL.

"The health department and the community needed to be directly in contact with each other," said Rich D'Amato, a spokesman for AOL. "We thought the responsible thing to do was to put the two together."

Tom Rielly, founder and chairman of PlanetOut, saw that his company, which is known and trusted by gays online, could lead an effort to explain the risks to members of the chat rooms and his general audience.

"This is the first application I know of doing disease control using the Internet," said Rielly. "We used every tool in our arsenal to reach the affected population."

Klausner said that since the campaign started, about twice as many men have gone to area clinics to be tested. Rielly helped Klausner draft a letter that was sent to the 99 screen names identified by the eight infected men as people who could have either given the disease or contracted it from them. The letter explained that any person having had sex with another member of the SF M4M group could have been exposed to the disease and should be tested.

In addition, PlanetOut staff members monitored the chat rooms, sending the letter to anyone who wanted information and answering specific questions.

The company also put up a prominent link on explaining the disease and giving contacts for treatment and clinics.

"I just said I'm going to take charge of this and have PlanetOut own this issue," said Rielly.

The entire situation could have been a privacy disaster. But because neither AOL nor PlanetOut ever knew the screen names revealed by the infected men (only the public health department had them) no one has raised any privacy concerns.

Internet users "are hypersensitive about privacy, especially in the gay community," said Rielly.

The health department has not shared the names of the affected men with anyone outside the agency, said Klausner.

Klausner said about a third of the 99 men identified as possibly being infected have been screened, and seven of them have been found to have syphilis.

The relationship that started a couple of weeks ago between the health department and PlanetOut will continue. Klausner plans to write a regular column for the Web site on sexually transmitted diseases.