You’ve probably heard of MRSA, which is pronounced “mersa” and stands for methicillin-resistant Staphylococcus aureus — a strain of bacteria that is resistant to methicillin, as well as pretty much every other antibiotic out there. MRSA is an example of evolution by natural selection — what didn’t kill its ancestors made them stronger, spawning a drug-resistant strain.

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There are drug-resistant strains of gonorrhea, trichomoniasis, and syphilis.

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Evolution is the force behind life’s diversity. Normally, diversity is a good thing — but when it comes to microbes that cause diseases like gonorrhea, trichomoniasis, and syphilis, these organisms’ ability to evolve new defenses against our antimicrobial drugs isn’t good for us.

STDs have plagued us for millennia, but it wasn’t until the 20th century that we finally developed antibiotics, which gave us a powerful tool against many of our most formidable sexually transmitted foes. Suddenly, scourges like gonorrhea and syphilis could be quickly and easily treated with a dose of penicillin.

Problem solved, right? Nope. Enter evolution by natural selection.

Whereas mammals reproduce sexually, bacteria clone themselves asexually. When you think of diversity, you don’t normally think of clones, but bacteria have a few tricks up their sleeves. First, while it takes a human a good 20 years to reproduce itself (give or take), bacteria can birth the next generation in 20 minutes (give or take). Their rapid proliferation also increases the chances that just one in a billion of them will evolve some freak mutation that will give rise to a new bacterial strain — perhaps one that can more easily withstand an onslaught of antibiotics. Second, some bacteria, such as those that cause gonorrhea, can transmit genes to one another, including genes that endow them with drug-resistant superpowers. A bacterium in possession of an antibiotic-resistance gene is at a competitive advantage.

So, just as the fastest cheetah will catch more prey and be more likely to pass its speedy genes to more progeny, the bacterium with the most defenses against antibiotics will thrive in an environment in which its antibiotic-susceptible brethren are felled by drugs — and be more likely to reproduce and replicate its genes. So it shouldn’t come as a surprise that there are antibiotic-resistant strains of STDs out there. We’ll focus on three of them.

Gonorrhea

Gonorrhea has been with us since antiquity, and while it often doesn’t have symptoms, it can cause painful urination, vaginal or penile discharge, painful bowel movements, and itching. Left untreated, it can lead to pelvic inflammatory disease in females and epididymitis in males, both of which affect fertility. Additionally, gonorrhea increases one’s chances of contracting or transmitting HIV.

The bacteria that cause gonorrhea have a special talent for developing resistance to antibiotics, and the emergence of antibiotic-resistant gonorrhea is one of the most pressing problems in infectious disease. In 2013, the Centers for Disease Control and Prevention named it an “urgent threat.” We have one remaining first-line gonorrhea treatment left: extended-spectrum cephalosporins, which include cefixime, which is taken orally, and ceftriaxone, which is administered as a shot. Resistance to cefixime was first documented in 1999, leaving ceftriaxone as our best remaining option, and the CDC’s first choice for treating gonorrhea. There are no good alternatives to ceftriaxone, which is why reports of ceftriaxone-resistant gonorrhea are deeply troubling.

Trichomoniasis

Trichomoniasis is caused by a single-celled parasite, and is the most common curable STD in the country. Symptoms include vaginal discharge, penile burning or discharge, spotting, and itching or swelling in the genital area — but around 70 percent of trichomoniasis infections are asymptomatic.

Fortunately, trichomoniasis can be cured by metronidazole or tinidazole, both of which are in the 5-nitroimidazole drug family. Unfortunately, resistance to these drugs is emerging, although so far, completely drug-resistant trichomoniasis is not a major problem. But what if trichomoniasis doesn’t go away after the first treatment? Drug-resistant strains of trichomoniasis aren’t necessarily incurable — they just might need to be treated with longer courses of medications or different combinations and dosages of drugs. However, the fact that trichomoniasis drugs are chemically similar makes some researchers wary, because if resistance continues to develop, we might not have viable alternatives if it turns out we need them.

Syphilis

Before good treatments were developed a century ago, syphilis was the most feared STD out there. Its initial symptoms, if present, can include a painless sore filled with a highly infectious liquid. Three to six weeks after the sores appear, other symptoms may appear, including lesions and rashes — usually on the soles of the feet or the palms of the hand. An unfortunate 15 percent of people infected with untreated syphilis reach the late stage, which can occur up to 20 years after initial infection. The late stage of syphilis includes severe damage to the nervous system, brain, heart, or other organs, and can be fatal.

First, the good news: A shot of penicillin still cures syphilis, and it is the recommended treatment in the United States. The bad news is that resistance to another class of drugs, macrolides (including erythromycin, azithromycin, and spiramycin), is arising — meaning that anyone who is unable to take penicillin might be at risk for a stubborn infection. Erythromycin failures have been documented since the 1970s, and azithromycin failures were noted more than a decade ago in San Francisco. Most recently, in 2009, Czech researchers reported a mutation giving syphilis bacteria resistance to spiramycin.

Luckily, penicillin remains the go-to drug for syphilis, and its continued susceptibility to penicillin after 70 years of use suggests that penicillin resistance requires a complicated series of mutations. The probability of that happening is deemed low.

Gonorrhea, trichomoniasis, and syphilis can still be cured with antimicrobial drugs — but an ounce of prevention is worth a pound of cure! To reduce risk, sexually active people should use condoms and dental dams, know the STD status of sexual partners, get tested regularly, and be monogamous or limit partners. You can be tested and treated for STDs, or pick up condoms, at any Planned Parenthood health center.

Click here to check out other installments of our monthly STD Awareness series!