Gonorrhea mutates in the pharynx, making oral sex far more risky than people think. Illustration by Chad Hagen

In January, 2009, a thirty-one-year-old prostitute visited a clinic in Kyoto, Japan, for a routine checkup. Because sex workers are so likely to acquire sexually transmitted diseases, many have themselves checked for infections even in the absence of symptoms. Indeed, although the woman displayed no outward signs of gonorrhea, her lab test came back positive; she carried the gonococcus microbe in her throat, a common reservoir. After a second visit, doctors at the clinic gave her an injection of ceftriaxone, an antibiotic considered by infectious-disease experts to be the definitive treatment for gonorrhea. It didn’t work; two weeks later, when she returned to the clinic, a throat culture again tested positive. She was given another dose, but it, too, failed, and, at first, doctors assumed that she had been newly infected. Now, however, public-health experts view the Kyoto case as something far more alarming: the emergence of a strain of gonorrhea that is resistant to the last drug available against it, and the harbinger of a sexually transmitted global epidemic. “The microbe appears to be emerging as a superbug,” Dr. Magnus Unemo, the head of the World Health Organization’s Collaborating Center for Gonorrhea and Other Sexually Transmitted Infections, in Sweden, told me recently. “This is what we have feared for many years.”

Gonorrhea is the second most commonly reported infectious disease in the United States, after chlamydia. More than three hundred thousand new cases are reported each year to the Centers for Disease Control, although the actual incidence is probably twice as high. A hundred million annual cases are estimated worldwide. Symptoms, when they occur, are very painful: swelling and a burning sensation in the urethra or the urinary tract, often accompanied by the release of pus. Untreated, gonorrhea in men can lead to scarring of the urethra and, eventually, to epididymitis, a painful condition of the testicles that can result in sterility. In women, the infection can migrate from the cervix into the uterus and the fallopian tubes, causing pelvic inflammatory disease and infertility. Infants born to mothers with gonorrhea can contract the infection in the eye and become blind. In some cases, among both men and women, the microbe enters the bloodstream, infecting the joints, the skin, the heart valves, and even the brain; it also increases one’s susceptibility to H.I.V. Remarkably, more than fifty per cent of women infected with gonorrhea display no symptoms at all; they carry and transmit it unaware. While gonorrhea in the throat may cause soreness, ninety per cent of throat infections, like the Kyoto case, produce no symptoms whatsoever.

Gonorrhea has been recognized since antiquity. The second-century physician Galen, mistaking pus for semen, derived the name from the Greek words gonos (“seed”) and rhoia (“flow”). In the time of the Tudors, gonorrhea was called “the clap,” a term that was still in use when, in 1760, at the age of nineteen, James Boswell, the journalist and the biographer of Samuel Johnson, was first infected by a London prostitute. He apparently contracted the disease at least a dozen more times; the infection spread to his testicles and prostate and scarred his urinary tract. (In his diary, he referred to his malady as “Signor Gonorrhea” and “a memorandum of vice.”) He died, at fifty-four, of kidney failure brought on, some medical historians believe, by complications from the disease. In 1901, the New York City medical examiner, the founder of a “social hygiene” movement, estimated that eighty of a hundred men in the city had contracted gonorrhea at some point in their lives.

Cures ranged from the absurd to the excruciating. At various times, Boswell tried bloodletting, a low-calorie diet, and a daily bottle of Kennedy’s Lisbon Diet Drink, which cost a lot, contained mostly sarsaparilla, and did nothing. In some cases, a doctor might use a thin tube to open the blocked urethra and, with a syringe, inject a solution of vitriol—sulfuric acid—or salt solutions made with mercury or lead, which over time could do as much damage as the disease. The historian Allan Brandt describes a cure proposed by one physician for chordee, a complication of gonorrhea that causes the head of the penis to curve downward. The affected member, the physician wrote, should be placed “with the curve upward on a table and struck a violent blow with a book . . . and so flattening it.”

In the nineteen-thirties, antibiotics changed the clinical picture of gonorrhea and other sexually transmitted diseases, and, with it, social attitudes. Once feared for its devastating complications, gonorrhea was now viewed as a bothersome but temporary price to pay for sexual freedom. The sexual revolution of the nineteen-sixties ushered in rising rates of gonorrhea, as condoms, which effectively prevent transmission, were abandoned in favor of oral contraceptives. Only after the risk of death from AIDS began to increase, in the nineteen-eighties, did condom use again become a norm. A federally funded gonorrhea-control program, started in 1972, perhaps made a difference; by 1997, the number of yearly cases of gonorrhea reported to the C.D.C. had fallen by nearly three-quarters compared with its peak, in 1975. In 2009, the number of gonorrhea cases in the U.S. was at an all-time low. “Ten or fifteen years ago, we thought it was going to be eradicated in some Western countries,” Unemo told me.

But as modern medicine has adapted so has the microbe. Natural selection has given rise to strains of the bacterium that are resistant, in varying degrees, to some or all of the treatments applied to them—sulfa drugs, penicillin, tetracyclines, fluoroquinolones, and macrolides. Now only one class of drugs, called cephalosporins—cefixime, a tablet, and ceftriaxone, administered by injection—is known to reliably treat it, and for several years resistance to cefixime has been rising. (In the lab, resistance is measured by testing how susceptible the microbe is to various concentrations of a drug.) Between 2000 and 2010, the number of cases of decreased cefixime susceptibility in California and Hawaii rose from zero per cent to more than four per cent and seven per cent, respectively, probably as a result of traffic from Asia, where cefixime resistance is more widespread. Five per cent is cause for concern; in August, the C.D.C. recommended phasing out cefixime nationwide and, instead, treating gonorrhea with a combination of ceftriaxone and either azithromycin or doxycycline. According to a recent British report, last year eleven per cent of isolates of the microbe showed reduced susceptibility to cefixime; among gay men, the figure is seventeen per cent.

“We are seeing decreased sensitivity to cefixime in all twenty-one countries in Europe,” Dr. Catherine Ison, a researcher in the U.K.’s surveillance program for sexually transmitted infections, told me. “It’s worrying.”

The Kyoto case, in 2009, marked the appearance of a microbial strain that was resistant to ceftriaxone—the first instance of broad resistance. In June, 2010, a second case emerged, in France; a third appeared in Sweden in July of that year, in a man who had recently had protected vaginal sex and unprotected oral sex with a casual partner in Japan. A fourth case occurred in Slovenia last September, and a fifth and sixth in Spain this past May. All appear to be descendants of a single cefixime-resistant strain, and they “are probably only the tip of the iceberg,” Unemo said. “Japan has been the epicenter for their emergence, and now these antibiotic-resistant gonococcal clones are spreading.” No cases have yet been reported in the U.S., but resistant gonorrhea is likely to arrive and spread long before physicians and the C.D.C. recognize it; some public-health officials predict that in five to eight years the superbug will be widespread. Whatever freedoms were won during the sexual revolution, bacterial evolution promises soon to constrain.