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The online banter was too steamy to ignore.

Gregory Johnson, a soft-spoken young man using the screen name “Adonis,” talked up sex to several hundred of his closest Facebook friends this spring. Once he had their attention, he sent a racy snapshot of two square wrappers tucked into his underwear along with a plea: Why not use a condom?

It was only a matter of time before the social media that keeps friends and family connected and amused was pressed into public service. Just as antismoking ads have come to saturate the airwaves, a flurry of personalized messages promoting H.I.V. testing and protected sex have popped up on thousands of smartphones, iPads and laptops in recent months.

The online campaign is the work of an unusual health project in the Bronx, which seeks to harness social media to educate gay and bisexual men about the risks of contracting H.I.V. and AIDS. The project, called “theSEXword,” recruited seven men and one woman who is transgender to build an online forum for sharing safe-sex messages with people who would never bother to pick up a brochure. It was financed with $25,000 from the Center for AIDS Research, a joint program of Albert Einstein College of Medicine and Montefiore Medical Center in the Bronx.

“It’s one on one; it’s directly to them,” said Kamari Perkins, 22, one of the eight cyber messengers. “They’re more receptive to someone they know than someone they don’t.”

In New York City, there were 3,404 newly diagnosed cases of H.I.V. in 2011, of which Brooklyn had the highest number (983), followed by Manhattan (866) and the Bronx (662), according to city health statistics. In total, 51 percent of all newly diagnosed cases were among men who reported having sex with men.

“The objective was to reach this hard-to-reach population and do it in an efficient way,” said Dr. Viraj V. Patel, a doctor at Montefiore, who oversaw the project with Dr. David W. Lounsbury, a medical researcher at Einstein. Montefiore currently treats about 5,500 people with H.I.V. in outpatient programs, for a total of more than 15,500 people since 1997.

The doctors turned to Sage Rivera, 31, a community leader, who tapped people for the project with connections to different crowds, from recent immigrants to those dabbling in subcultures like Goth and graffiti. The group of eight, called peer leaders, range in age from 15 to 27 and have varied backgrounds and interests.

For instance, Mr. Johnson, 24, is popular in the cyberworld with more than 20,000 followers combined on social media sites, including Facebook and Twitter. Another is a dancer, and still another, a gay rights activist. As Mr. Rivera put it, arguments erupt all the time because “they don’t speak the same, they don’t have the same history.”

In a basement computer lab in a Montefiore building, the peer leaders have met weekly to share ideas and tips for getting their messages heard. They are paid a nominal stipend of $35 per meeting, plus travel expenses.

“Don’t make it too preachy,” Mr. Johnson advised. “You just want to make it cool. You want to speak the way they speak.”

Mr. Johnson explained that he engages in the sex talk because if he just posted a message about H.I.V., his friends would skip over it. “I have a lot of friends who are H.I.V. positive, so I feel like I’m actually doing something to help,” he said.

By the project’s count, the peer leaders have reached more than 50,000 people through social media. They have received back more than 1,600 responses (likes, comments, retweets). At their request, their friends and acquaintances have also completed nearly 300 online surveys about their individual H.I.V. testing history to collect data for research.

Teddy Reyes, 18, said that since he joined the project, he has helped persuade a dozen of his friends to get tested for H.I.V. On a recent afternoon, he began his online campaign by posting 20 funny slogans about condoms on Tumblr, including No. 6: “No glove, no love.”

“People don’t want to talk about it because it’s so serious,” he said. “But if you make it funny, that gives people a way to feel comfortable about it.”