The outbreak began in Leeds, England, in March, when doctors confirmed 12 cases of a worrisome strain of gonorrhea. Dubbed "super-gonorrhea," the sexually transmitted infection wouldn't respond to the antibiotic azithromycin, one of the two main drugs used to treat the STI.

The outbreak may be far from over.

In a letter issued to doctors and pharmacies across Britain, chief medical officer Dame Sally Davies warned that gonorrhea has "rapidly acquired resistance to new antibiotics," according to the BBC.

The potential outcome is scary. If gonorrhea bacteria continue developing antimicrobial resistance, the STI, according to the chief medical officer, "is at risk of becoming an untreatable disease."

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So what is gonorrhea, anyway? Gonorrhea is a sexually transmitted infection caused by the Neisseria gonorrhoeae bacterium. It's capable of occurring in both males and females, according to the Centers for Disease Control and Prevention. Infection can occur in the urethra and in body parts with mucus membranes, such as the mouth, throat, eyes and rectum. In female-bodied people, that also includes the cervix, uterus and fallopian tubes.

Here in the United States, 350,062 cases of gonorrhea were reported to the CDC in 2014. But that's likely less than half the number of infections that take place; given the huge number of cases that go undetected and unreported, the CDC estimates that 820,000 new gonorrheal infections occur in the U.S. each year.

You can contract gonorrhea through skin-to-skin sexual contact with an infected person's penis, vagina, mouth or anus, the CDC says, noting that ejaculation doesn't have to occur for the STI to be transmitted.

Many people with gonorrhea experience no symptoms. For males who do, symptoms include white, yellow or green urethral discharge and testicular pain. For females, symptoms include vaginal discharge and bleeding between periods.

If left untreated, gonorrhea can cause pelvic inflammatory disease in females, the CDC warns — the complications of which include infertility. It can also prevent males from being able to father children. Sometimes, it can even spread to the blood or joints, which can be fatal.

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How is gonorrhea treated, and why is it becoming resistant to those drugs? There are a number of ways bacteria can become resistant to drugs. One occurs when a patient takes a too-mild dose of medication or doesn't complete a full course of antibiotics. The weaker bacteria are killed, but the resistant bacteria survive.

Gonorrhea tends to be treated in two steps: an injectable drug called ceftriaxone, and an oral drug — either azithromycin or doxycycline. As of 2012, the CDC recommends ceftriaxone, plus either azithromycin or doxycycline, as "the only first-line treatment" for gonorrhea. If treated correctly, the STI can clear up in a couple of weeks. But a BBC investigation in March found that a number of healthcare consultation websites were only offering azithromycin — not ceftriaxone, too.

In the "super-gonorrhea" outbreak in Leeds, patients were exhibiting a resistance to azithromycin, the BBC reported. Doctors are also wrongly prescribing ciprofloxacin — a drug to which gonorrhea bacteria are widely resistant, and which hasn't been recommended in years, according to the Washington Post.

In her letter to Britain's medical professionals, Davies called for everyone to follow proper protocol for the treatment of gonorrhea. The more patients who don't complete full course of treatment, the more resistant gonorrhea bacteria survive. So it was "extremely important that suboptimal treatment does not occur," the letter said.

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Warnings of antibiotic-resistant gonorrhea are nothing new. A CDC report on antibiotic resistant threats in the U.S. found there were 246,000 cases of drug-resistant gonorrhea infection in 2013. That's 30% of the estimated 820,000 total annual cases. The report even classified the bacteria's hazard level as "urgent," the most severe of three possible ratings.

Bacteria classified as "urgent" "may not be currently widespread but have the potential to become so and require urgent public health attention to identify infections and to limit transmission," the report said.

In September 2014, the U.S. government published "National Strategy for Combatting Antibiotic-Resistant Bacteria," including Neisseria gonorrhoeae. The report stated that some gonorrhea bacteria are developing resistance to ceftriaxone, the gonorrhea-fighting injectable drug.

The report also estimated the public health impact over a 10-year period of widespread ceftriaxone-resistant gonorrhea bacteria. It included 75,000 additional cases of pelvic inflammatory disease, 15,000 cases of epididymitis, and medical costs of at least $235 million.

"As more strains of bacteria become resistant to an ever-larger number of antibiotics, our drug choices will become increasingly limited and expensive and, in some cases, nonexistent," the report said. "If this trend continues unchecked, a wide range of modern medical procedures, from basic dental care to organ transplants, likely would be accompanied by a much greater risk of developing a difficult-to-treat or untreatable antibiotic infection."

Dr. Vanessa Allen with a plate of cultured Neisseria gonorrhoeae. Richard Lautens/Getty Images

Is there hope? One way to limit antibiotic-resistant strains of gonorrhea is for people not to get gonorrhea in the first place. The CDC recommends the same strategies you probably learned in high school sex education: mutual monogamy, correct condom use and, yes, abstinence.

And when people do become infected with gonorrhea, it's imperative we stop it from spreading. That means detecting and treating the STI as quickly and effectively as possible. "Screening, rapid detection, prompt treatment and partner services," the CDC says, "are the foundations of gonorrhea control in the United States."