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On Tuesday, the Centers for Disease Control and Prevention announced that reported cases of three sexually transmitted diseases in the United States had reached an all-time high in 2017. Rates of gonorrhea rose by 67 percent, syphilis by 76 percent, and chlamydia by 21 percent, to a total of almost 2.3 million cases nationwide. According to the CDC, 2017 surpassed 2016 as the year with the most reported STD cases on record—and marked the fourth year in a row that STDs increased steeply in the U.S.

It might seem logical that higher STD rates would go hand in hand with increased sexual activity, but a flurry of recent research indicates American adults are actually having less sex on average than they have in decades.

Often-cited data from the biennial General Social Survey, for example, indicate that the number of Americans who haven’t had sex at all in the past year has risen from 18 percent to 22 percent over the past two decades, while the number of Americans between the ages of 18 and 30 who report having sex twice a month or more has declined from almost three-quarters in the early 2000s to two-thirds by 2016. And a study published in the Archives of Sexual Behavior in 2017, authored by a research team led by Jean Twenge, found that American adults were engaging in sex nine fewer times a year on average than they did in the late 1990s. So if there’s less sex being had nationwide, how are so many more people getting STDs?

The CDC’s study on STD rates in the United States in 2017 will be published in full this fall, at which time more details, such as demographic information about who is most affected, will be available. But when I asked Gail Bolan, the director of the CDC’s Division of STD Prevention, how exactly these two trends in sexual behavior were happening at once, she explained that the sexual behaviors known to be contributing to widespread STD transmission have almost nothing to do with how much sex people are having—rather, it’s riskier sex that’s being had.

Two factors Bolan identifies as potentially contributing to the record-high rates of reported STDs are a rise in condom-less sex and a rise in high-risk sexual behaviors associated with opioid use and addiction.

“Among gay and bisexual men as well as heterosexual [couples], there are some behavioral surveys that are suggesting that condom use is on the decline,” Bolan says. Among communities of men who have sex with men, she says, condom use has been on the decline for several years: “A lot of that was related to when lifesaving HIV medications became available … Those are great tools for preventing HIV transmission, but they have no impact on these other STDs.”

More recently, though, Bolan and her team have seen changes in transmission patterns. Over the past ten years, she says, condom-less sex has increased across demographics. “We’re starting to see increases [in STDs] in heterosexual people, especially women and pregnant women, which means these infections are getting into new populations.”

Additionally, Bolan says, other CDC research suggests a link between STD transmission and the risky sex acts often associated with opioid use and addiction. She cites a soon-to-be-published CDC study that found 15- to 24-year-olds who reported injected drug use in the past year were more likely to be diagnosed with chlamydia, syphilis, and gonorrhea than those who didn’t inject drugs. More importantly, she adds, “injecting drugs was also associated with higher rates of forced sex, sex with people who exchange money or drugs for sex, and sex with other people who inject drugs”—which are all “high-risk factors” for STD transmission. (Chlamydia, gonorrhea, and syphilis are really only transmitted through sexual activity, not through blood exposure from sharing needles.)

And as Angela Lashbrook recently pointed out in The Atlantic, another reason STD rates are climbing is because funding cuts have been affecting public-health centers and STD-prevention and -treatment programs throughout the United States in recent years. “When underfunded, these initiatives decrease their hours and staff, and sometimes close altogether, making STD detection and treatment more difficult,” she writes.

The CDC’s findings, coupled with the decline in the frequency of Americans’ sexual encounters, indicate that Americans’ sex lives are changing; when people are doing it, they’re doing it with fewer precautions in place, and with less of a safety net for diagnosis and treatment. As sex has fallen further down the priority lists of many Americans, so perhaps has sexual safety.

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