(CNN) -- When Jennifer Nicholas sees television shows or movies where characters "hook up" or have sex with "friends with benefits," she cringes, because that's how she got herpes.

"Getting an STD wasn't even something that crossed my mind," said Nicholas, 39, who learned that she had herpes at age 22. "One day I'm at the doctor's office and it was, 'Surprise! You've got herpes.' "

Experts in sexually transmitted diseases say they've become increasingly concerned about the trend toward having what they call "sexual involvement in nonromantic contexts" -- the technical term for hookups or "friends with benefits" -- because they're especially likely to spread sexually transmitted diseases.

The concern is that that people who have nonromantic relationships tend to have several partners at one time -- "concurrency," in sexual behavior lingo -- in contrast to people engaged in romantic relationships, who tend to be monogamous for the duration of the romance.

"We're concerned that concurrency is speeding up the transmission of sexually transmitted diseases," said Tony Paik, an assistant professor of sociology at the University of Iowa who recently published a study on the subject.

"This is a direct route for spreading STDs. There are important implications here for public health," he added.

In Paik's study, published last month in the journal Perspectives on Sexual and Reproductive Health, he found that 17 percent of men and 5 percent of women had at some point had more than one sexual partner at a time. Seventeen percent of women and 8 percent of men said they'd been exclusive but their partner had not.

For both genders, having sex with a friend made someone less likely to be monogamous.

"Sex with the ex"

Peggy Giordano, a professor of sociology at Bowling Green State University, studies the sexual behavior of young people, and she's also concerned about the phenomenon of having sex in nonromantic ways.

"It seems more acceptable now to have nonromantic sexual encounters," said Giordano, who's studied the sex lives of 1,300 teens and young adults in Lucas County, Ohio. "When there's no romance, there's no basis for demanding fidelity from the other person."

She says it's not just the number of partners at one time; it's that people's behavior seems to be different when they're having "friendly" sex in contrast to romantic sex.

When people have sex with a friend, they tend to be more trusting that the person doesn't have a sexually transmitted disease and therefore fail to use a condom, she says.

"If you've known a person for a while, you don't have that vigilance. You're probably not going to ask them to go and get tested for STDs," Giordano said. (To find out whether you should get a test for an STD, you can take this quiz.)

The U.S. Centers for Disease Control and Prevention can help you find a testing site near you.

This lack of vigilance about STDs is especially true when the sexual partner is a former boyfriend or girlfriend, she adds.

"We're finding that 'sex with the ex' is a very common experience," said Giordano, who's been studying the group of Ohio youth since 2001.

"It's seemingly safe, since they used to be your girlfriend or boyfriend. But of course you don't know what they've done since you broke up. You don't know their full portfolio of partners," she said.

What are the chances?

Through her work with the Atlanta H Club, a social and support group for adults with herpes or the human papillomavirus, Nicholas is now more aware of the chances that a prospective partner could have a sexually transmitted disease.

It's impossible to say precisely what the chances are you'll catch an STD from any one person, but there are studies that can give you a clue. One important factor to keep in mind: For biological reasons, women are more likely to catch an STD from a man than vice versa.

Human papillomavirus

HPV is the most common sexually transmitted disease in the United States, according to the Centers for Disease Control and Prevention.

About one in four U.S. females age 14 to 59 has HPV, according to a 2007 study in the Journal of the American Medical Association. The HPV rate was highest -- 44 percent -- for women ages 20 to 24.

HPV is not as common in men, according to a 2006 article in the Journal of Infectious Diseases, but is still "highly prevalent." The study, which looked at 40 studies on HPV and men, found that 56 percent of the reports found that at least one in five men had HPV.

Herpes

Nearly one in five Americans has herpes simplex virus, according to a 2006 study in the Journal of the American Medical Association. The rates for women were higher than for men: 23 percent, compared with 11 percent. Rates were especially high among African-Americans.

Gonorrhea

Your chances of getting gonorrhea from a sexual encounter are significantly lower than your chances of getting HPV or herpes. A 2007 study in the Annals of Internal Medicine showed a 0.24 percent prevalence rate. Rates were highest among teens but still less than 1 percent.

Chlamydia

Nearly half of the people in the above study who had gonorrhea also had chlamydia, but again, infection rates were significantly lower than for HPV and herpes.

According to a 2007 report in the Annals of Internal Medicine, 2.2 percent of Americans ages 14 to 39 had chlamydia. The rates were highest for teenage girls (4.6 percent) and for black women (7.2 percent).

CNN's John Bonifield contributed to this report.