Thanks to the sex-tech revolution—and the hookup-app industry it has birthed—more people are having more on-demand sex, free of strings, accountability, and knowledge of a partner's name and sexual history. The by-product of this blissful ignorance: STDs, and lots of them. According to the Centers for Disease Control and Prevention, the number of cases of chlamydia, gonorrhea, and syphilis—the so-called Big Three—is rising fast. New York City's Chelsea neighborhood recently laid claim to the highest syphilis-infection rate in the country, and health departments in Rhode Island and Utah blamed Tinder and Grindr, in part, for their states' epidemic levels of STDs. The alarm has sounded in the U.K. as well—Peter Greenhouse, of the British Association for Sexual Health and HIV, recently declared, "You don't have to be a genius to work out that these sorts of apps make having casual sex a damn sight easier. Thanks to Grindr or Tinder, you can acquire chlamydia in five minutes."

Max, a 24-year-old clothing designer in Tempe, Arizona, is vigilant about STDs—especially after his best friend recently contracted chlamydia, twice, from Tinder dates. But Max occasionally lets down his guard after a few drinks or if the hookup is really hot. "Women tend to push using a condom if it's a one-night stand," he says. "But if it's our second encounter, they'll almost never ask for one. If we continue seeing each other, they'll sometimes ask not to use one." Most, he says, rely on oral contraception or the morning-after pill. But regardless of the risks, there's more opportunity than ever.

"I probably get laid 10 times more often via an app than when I'd just try to meet someone at a club," Max says. This shouldn't come as a shock. He has 20 different hookup apps running on his phone: DateHookup, MeetMe, Moonit, Jaumo, I-Am, Parlor, Grouper, SpeedDate, Hot or Not, Fling, Flurv, Lulu, Razzou, Skout, Tinder, Tagged, Happn, Down, iHookup, and his current favorite, Plenty of Fish.

That may sound excessive, but Max is hardly alone: The Pew Research Center estimates that one in ten Americans is using hookup apps, and a new study published in the Archives of Sexual Behavior found that four in ten report having had casual sex in the past year. For the young, tech-savvy urban dweller, the rates would seem to be much higher.

"The thing with these apps is that when people set up a profile, they may or may not be truthful, and they may be switching it all the time to maintain some anonymity," says Adina Nack, Ph.D., a medical sociologist and sexual-health educator. "In a situation where there's no trust between partners and anonymity is valued, the odds of disclosing an STD are pretty low."

Cari, a 25-year-old in New York City who describes herself as "very sexually active," says she generally skips protection—after all, condoms are uncomfortable and stopping to put one on "messes up the flow." Recently, she began sleeping with a guy in his forties. "I didn't want to seem young and naïve by asking him to wear one, so I just ended up ignoring the conversation altogether," she says. "I assume at that age, you know whether or not you have any diseases."

When sex is largely negotiated by text message, open dialogue is discouraged. Even if the person you go home with knows he or she is infected—and many don't, since chlamydia, gonorrhea, and syphilis can all live in the genitals, throat, and anal area for months or longer, symptom-free—hooking up via app doesn't exactly foster a sense of openness or accountability. Perhaps you want to tell the woman from last weekend that you're going to the doctor, but you don't know how to reach her or even if Stacy's her real name.

Steve, a 41-year-old risk manager, meets many of his partners through Ashley Madison, a site dedicated to facilitating extramarital affairs. And that has an advantage: Almost all of the women he sleeps with are on the pill ("They take care of protection so their husbands don't find out," he says). Steve doesn't ask whether they've been tested for STDs, and they don't ask him. "I don't know how I would bring that up," he says.

"The STD conversation is never a comfortable one," says David Malebranche, M.D., who sees plenty of cases at the University of Pennsylvania's Student Health Service. "And it's really not a conversation they want to have if they're just trying to get a nut."

And for some guys—itching and burning notwithstanding—STDs are simply no big deal. After all, unlike HIV (the ultimate BFD) or herpes (which stays with you forever), the Big Three can usually be treated with a quick course of antibiotics. A week later, you're back online. As Keith, a thirtysomething New Yorker in publishing who got chlamydia and gonorrhea in his twenties, says, "No younger people on these apps use condoms. 'Gonorrhea is curable, so why not?' That's the reasoning."

But STDs haven't survived into the age of the superbug without adapting, and the CDC reports that new strains of the Big Three are becoming increasingly resistant to available drugs. "The cures we had are no longer consistently working," says Nack. "We've reached a point where an infection is actually not deviant—it's the norm. If we all walked around with the assumption that the person you're about to get naked with has an STD, I think people would act very differently." Starting, perhaps, with the swipe of a finger.