The center’s director, Amy Ellenbogen, had decided several years earlier to begin focusing on gun violence in the African-American community. It took her three years to get financing — it came from the stimulus — at which point she began to investigate potential models for reducing shootings. When she saw what Slutkin was doing, she got excited: “This made so much sense to us,” she says.

Just as with epidemics, the key is intervention. The first step in combating an epidemic is to find the disease carriers. Save Our Streets hired what it called “outreach” staff — who had once been prone to gun violence themselves — whose job it was to identify the people in the community most likely to commit gun violence. Then they developed a relationship with them, with the goal of dissuading them from using a gun.

A second group of staff members are called “violence interrupters.” They try to mediate when something happens that could cause someone in the neighborhood to shoot someone else. Finally, an S.O.S. staffer has the task of going to the hospital whenever a shooting takes place. His role is to convince both the person who has been shot and his family and friends not to exact revenge with a gun.

When I expressed some mild skepticism that gang members would listen to this kind of counseling, one of the outreach workers quickly replied: “We have influence because we have lived that life.”

There is a second reason the interventions work. “If you are a gang member, your No. 1 fear is getting shot,” says Daniel Webster, the co-director of the Johns Hopkins Center for Gun Policy and Research. Webster has studied the public health approach in Baltimore. His belief is that programs like S.O.S. are effective because gang members are often looking for reasons not to resort to gun violence. “If you ask gang members to take a vote on whether someone who disrespects someone should be shot, the vast majority would answer no,” Webster said. “But they feel powerless to stop the behavior by themselves.” In Crown Heights, S.O.S. gives them the out they are looking for.