The STIs most likely to affect male fertility are chlamydia, gonorrhea, and Mycoplasma genitalium — all bacterial infections that can inflame the urethra and the tube behind the testicle. Each is often silent, so a man can carry one for months without symptoms. Because they overlap, a lab test identifies which one (if any) is present; the symptom picture won't.
Chlamydia trachomatis
Neisseria gonorrhoeae
Mycoplasma genitalium
| Item | Value |
|---|---|
| Chlamydia | curable — Chlamydia trachomatis |
| Gonorrhea | curable — Neisseria gonorrhoeae |
| Mycoplasma genitalium | curable — Mycoplasma genitalium |
Which STIs cause fertility problems in men?
Untreated bacterial STIs damage male fertility through the same general mechanism: the infection climbs from the urethra into the epididymis (the coiled tube behind each testicle where sperm mature and are stored). The resulting inflammation, epididymitis, can scar that tube and block the path sperm travel, while the inflammatory environment impairs sperm motility and quality. Three infections account for most of this risk.
Chlamydia
Chlamydia is caused by the bacterium Chlamydia trachomatis, and most US genital infections come from serovars D–K CDC Chlamydia. It usually has no pattern at all: it's a 'silent' infection, with about half of infected men reporting no symptoms. When symptoms do appear, they typically show up within one to three weeks of exposure as urethral discharge or burning on urination. An untreated infection can smolder and spread to the epididymis without ever announcing itself. Because reinfection is common after treatment, retesting matters; see chlamydia reinfection for why a repeat test weeks later is standard.
Gonorrhea
Gonorrhea, caused by Neisseria gonorrhoeae, can infect the genitals, rectum, and throat CDC Gonorrhea. In men it's somewhat more likely to make itself known than chlamydia: burning on urination and a white, yellow, or green penile discharge are the classic signs, and less commonly the testicles swell and become painful, which warns of epididymitis, the complication that threatens fertility. Even so, a man can have gonorrhea with mild or no symptoms, and then it goes untreated long enough to scar.
Mycoplasma genitalium (Mgen)
Mycoplasma genitalium is a bacterium that lacks a cell wall, which is why penicillins and cephalosporins simply don't work against it CDC Mgen. It's an emerging cause of urethritis in men, presenting as non-gonococcal urethritis — discharge and painful urination — that is persistent or keeps coming back despite treatment. Antibiotic resistance, especially to macrolides, is the central problem, so an Mgen infection that won't clear is the one most likely to linger and inflame the reproductive tract. If symptoms recur after you've been treated for something else, that recurrence is itself a clue — awareness is key in mg diagnosis and treatment.
How do you tell them apart?
You mostly can't by looking. The discriminating features are subtle and unreliable: gonorrhea tends toward a more obvious colored discharge, chlamydia leans silent or causes a thinner discharge, and Mgen shows up through urethritis that won't quit. These overlap too much to separate by sight, and all three are frequently asymptomatic. A man with no symptoms could have any of them or none. Only a test turns a guess into an answer.
| Feature | Chlamydia | Gonorrhea | Mycoplasma genitalium |
|---|---|---|---|
| Cause | Chlamydia trachomatis | Neisseria gonorrhoeae | Cell-wall–lacking bacterium |
| Symptoms in men | Often none; discharge or burning | Burning, colored discharge, swollen testicle | Persistent/recurrent urethritis |
| Often silent? | Yes — about half of men | Sometimes | Yes, especially over time |
| Timing of symptoms | Usually 1–3 weeks after exposure | Not stated by CDC reviewed here | Variable; recurrent |
| Fertility risk | Epididymitis from silent spread | Epididymitis, more overt | Lingering inflammation; resistance complicates clearing |
How is it tested?
All three are diagnosed by a nucleic-acid amplification test (NAAT), which detects bacterial genetic material rather than relying on symptoms. NAAT is the recommended method for chlamydia and the preferred test for gonorrhea, where it's highly accurate, and an FDA-cleared NAAT exists for Mgen on urine or a swab CDC STI Tx Guidelines, 2021. In practice that means a urine sample, a self-collected swab, or a quick exam depending on what's suspected — free or low-cost at health departments, Planned Parenthood, and Title X clinics, with results usually back in a few days. For the full how-to, see get tested, and read more on the gonorrhea test options. If you're timing a test around a specific encounter, check when to test after exposure so you don't test too early and get a false negative.
What to do next
If a test comes back positive, all three infections are treatable — chlamydia and gonorrhea with standard antibiotic courses, and Mgen with a regimen that works around its cell-wall and resistance issues, so a clinician picks the drug rather than you. Treat any current partners at the same time, avoid sex until everyone's finished and cleared, and plan to retest, since most apparent recurrences are reinfection rather than treatment failure. Treating before inflammation scars the epididymis protects fertility.
Red flags — when to get seen urgently
- Sudden swelling, pain, or tenderness in one testicle — this can signal epididymitis, and rapid, severe testicular pain can also mean testicular torsion, a surgical emergency that must be ruled out within hours.
- Fever with genital or pelvic pain, which suggests the infection has spread beyond the urethra.
- Discharge or burning that persists or returns after you've completed treatment, which points to reinfection or a resistant infection like Mgen.
- Blood in the urine or semen alongside any of the above.