Five years ago, David Anderson, then 18, knew that he was at high risk of HIV infection. But he had never had a test for the virus because he feared that people at any clinic he visited would make assumptions about his sexuality and status. “I'm a bit feminine,” says Anderson, who now openly identifies as both a gay man and a transgender woman (Davida). But then a friend told him about an unusual community center in Abuja, where he lives. “It was nice and safe for people of my kind to come,” he says. Anderson tested negative and remains uninfected.

The clinic, opened by the Institute of Human Virology, Nigeria, in 2012, was the first in the country to cater to men who have sex with men (MSM) and transgender people, communities so heavily stigmatized that treating them is technically illegal. The clinic nevertheless provides state-of-the-art HIV services and conducts some of the most authoritative research on those communities in Nigeria. “We offer one-stop shopping,” says Man Charurat, an epidemiologist heading a multifaceted research project at the clinic with virologist William Blattner, both of whom are based at the institute's main facility in Baltimore, Maryland.

The study is known as Trusted Community Center to Reduce HIV Infections by Engaging Networks of Friends and Partners to Support Safe Sex, or TRUST—the nickname of the clinic. The study looks at, among other things, the prevalence of HIV, the rate of new infections, and how the virus spreads in networks of sexual partners. Each person who enrolls is given a small incentive to bring five friends. TRUST—which has a second clinic in Lagos run by the U.S. Military HIV Research Program—now has more than 2000 participants, a startling 45% of whom tested positive for HIV.