More than a quarter of gay and bisexual men in some cities and states in the South are living with H.I.V., according to a new study — a far higher rate than in the country as a whole.

The study shows how much more common H.I.V., the virus that causes AIDS, is among gay black men, especially in the South, as well as how little is being done to prevent its spread in a group whose members face discrimination and are less likely to have medical insurance.

Gay men with insurance are more likely to be in treatment if they are infected or to be using pre-exposure prophylaxis if they are not, both of which significantly cut the chances that an infection will be passed on.

“There are just not enough services,” said Dr. Ronald O. Valdiserri, an AIDS expert at the Bloomberg School of Public Health of Johns Hopkins University, who wrote an editorial accompanying the study.