Mycoplasma genitalium (often called M. gen or Mgen) spreads through vaginal and anal sex with an infected partner. It’s a sexually transmitted bacterial infection that affects roughly 1% of the general population aged 16 to 44, though rates climb as high as 38% among people visiting sexual health clinics. What makes it particularly easy to spread is that the vast majority of infected people have no idea they’re carrying it.
Vaginal and Anal Sex Are the Main Routes
The bacteria pass from one person to another during unprotected vaginal or anal intercourse. Like other STIs, direct contact with infected genital tissue or fluids is what allows the organism to establish itself in a new host. Using condoms significantly reduces the risk of transmission.
Whether oral sex can spread Mgen is still an open question. The CDC notes that researchers are actively investigating this possibility but haven’t reached a definitive answer. This uncertainty means oral transmission can’t be ruled out, but vaginal and anal contact remain the clearly documented pathways.
There is no evidence that Mgen spreads through casual contact, shared towels, toilet seats, or swimming pools. The bacteria is fragile outside the human body and requires the kind of direct mucous membrane contact that happens during sex.
Most Infected People Have No Symptoms
This is the detail that catches most people off guard. In one study of men who have sex with men, 93% of Mgen infections were completely asymptomatic. That means someone can carry and transmit the bacteria for weeks or months without any sign that something is wrong. The CDC confirms that a person with Mgen can pass the infection to a partner even when they have no symptoms at all.
When symptoms do appear, they typically show up within 2 to 35 days after exposure. In women, this can look like unusual vaginal discharge, pain during sex, or bleeding between periods. In men, it often causes urethritis, with symptoms like burning during urination or discharge from the penis. But because these symptoms overlap with other common infections like chlamydia, Mgen often goes unrecognized even when it does cause problems.
Transmission During Pregnancy
There is limited evidence that Mgen can pass from a pregnant person to their baby during delivery. The bacteria has been detected in the airways and eye specimens of newborns born to infected parents. This type of vertical transmission appears uncommon, but it’s one reason infections during pregnancy deserve attention.
Who Is Most Likely to Get It
The risk factors for Mgen mirror those of other STIs. Having multiple sexual partners, inconsistent condom use, and concurrent infections all increase the likelihood of picking it up. The UK general population prevalence of about 1% might sound low, but it rises dramatically in higher-risk groups. Among people already seeking care at sexual health clinics, up to 38% test positive.
Despite how common it is, routine screening for Mgen in people without symptoms is not currently recommended by any major guideline. Testing is generally reserved for people who have symptoms of urethritis, cervicitis, or pelvic inflammatory disease, particularly when initial treatment for those conditions hasn’t worked. This selective approach partly reflects concerns about antibiotic resistance: the bacteria has developed high rates of resistance to first-line treatments, and widespread testing could lead to overuse of the limited antibiotics that still work against it.
How to Lower Your Risk
Condoms are the most effective tool for reducing Mgen transmission during vaginal and anal sex. Because the infection is so often silent, you can’t rely on a partner being symptom-free as a sign they’re uninfected. If you’re experiencing unexplained genital symptoms, especially after a new sexual partner, ask specifically about Mgen testing. Standard STI panels don’t always include it, and it requires a specialized test, typically a nucleic acid amplification test run on a urine sample or genital swab.
If you do test positive, your sexual partners need to be tested and treated as well. Reinfection from an untreated partner is common and contributes to the cycle of transmission. Treatment usually involves a short course of antibiotics, though the growing resistance problem means your provider may need to run additional testing to determine which antibiotic will actually work against your specific strain.