U.S. and Mexican researchers have found that high incidence of unprotected sex and intravenous drug use among male clients of female sex workers in Tijuana, Mexico, is driving rates of HIV and other sexually transmitted infections in the region, United Press International reports.

The study, conducted by a binational team of global health researchers at the University of California at San Diego’s School of Medicine, examined 400 clients—most of whom were Hispanic—with the average age of 37 years. They discovered that during the past year, male clients had sex with a female sex worker more than 25 times on average. More than half of the men reported having unprotected sex during the past four months; in addition, half reported that they had taken an HIV test. Out of all the clients examined, 14.2 percent tested positive for chlamydia, gonorrhea, syphilis and/or HIV.

“Targeted intervention among male clients is necessary to prevent the spread of HIV and other sexually transmitted infections—intervention that doesn’t solely place the onus on female sex workers,” said Thomas L. Patterson, the study’s lead author.

According to the study, the prevalence of HIV infection among clients was similar to that of female sex workers in Tijuana.