There is encouraging news in the fight against H.I.V in the United States: Over all, fewer people are contracting the virus, which causes AIDS. But H.I.V. continues to take a devastating toll on men who have sex with men, particularly among African-Americans and Latinos. If current rates persist, one in every six men who has sex with men will become infected with H.I.V., the Centers for Disease Control and Prevention said last week in its first lifetime risk analysis. One out of every two black men and one out of every four Latino men in this segment of the population is projected to become infected.

Officials at the C.D.C. say those trends can be reversed. Infection rates could drop dramatically if more people living with H.I.V. were treated with drugs that suppress the virus, which would vastly reduce the risk of contagion. If 80 percent of those who know they have the virus receive adequate treatment — a goal federal health officials have set — the agency estimates that 168,000 infections can be prevented over the next five years. Currently, less than half of the 1.2 million Americans infected with H.I.V. are receiving effective treatment.

Expanding access to the drug PrEP (pre-exposure prophylaxis) to people who are H.I.V.-negative but at high risk of becoming infected could prevent an additional 17,000 infections over the same period, the C.D.C. said.

Gay and bisexual men continue to be at high risk of getting the virus because many have unprotected sex with multiple partners. This appears to be particularly common among younger men who were not present in the early days of the epidemic, when infected men fell gravely ill and died soon after testing positive.