The AIDS epidemic in America is rapidly becoming concentrated among poor, young black and Hispanic men who have sex with men.

Despite years of progress in preventing and treating H.I.V. in the middle class, the number of new infections nationwide remains stubbornly stuck at 50,000 a year — more and more of them in these men, who make up less than 1 percent of the population.Giselle, a homeless 23-year-old transgender woman with cafe-au-lait skin, could be called the new face of the epidemic.“I tested positive about a year ago,” said Giselle, who was born male but now has a girlish hair spout, wears a T-shirt tight across a feminine chest and identifies herself as a woman. “I don’t know how, exactly. I was homeless. I was escorting. I’ve been raped.”