April 10, 2009 -- The phrase "sleeping with the enemy" has a whole new meaning.

A new study shows that a sexual partner’s activities may have more to do with whether or not you gets an STD than your own behavior.

Participants in the study included 412 black and white men and women ages 15 to 24 who attended an urban STD clinic in Pittsburgh. All reported having had heterosexual sex.

Each year, roughly 19 million people in the U.S. contract a sexually transmitted disease, says the CDC. About half are between the ages of 15 and 24.

Researchers measured six characteristics to gauge the risk of a sexual partner:

The partner had a problem with marijuana or alcohol.

The partner was at least five years older or younger.

The partner had been in jail.

The partner had had sex with other people in the past year.

The partner had had an STD in the past year.

Study participants were asked about their sexual characteristics and their partners’ characteristics. Among those whose partners’ activities were labeled as high risk, 53% were diagnosed with an STD. Among those whose own behavior was labeled as high risk, 38% were diagnosed with an STD.

The riskiest characteristics were age difference and if the partner had had an STD in the past year.

“If you are choosing high-risk partners, you are much more likely to have an STD, even when we account for your condom-use patterns,” Stephanie A.S. Staras, lead author and an assistant professor of epidemiology and health policy research in the University of Florida College of Medicine, says in a written statement.

“The theory is simple: You need to have sex with someone who has an STD to get an STD. Based on the prevalence of STDs in the United States, it seems like the public may not fully understand their risk.”