Trichomonas vaginalis spreads primarily through sexual contact. The parasite passes between partners during unprotected sex, moving from a penis to a vagina, from a vagina to a penis, or from one vagina to another. Unlike bacteria or viruses that cause other sexually transmitted infections, trichomoniasis is caused by a single-celled parasite that physically attaches to the lining of the urogenital tract and can survive there for months or even years if untreated.
How the Parasite Spreads During Sex
Trichomoniasis is almost always transmitted through genital-to-genital contact. The parasite lives in the lower genital tract in women, including the vulva, vagina, cervix, and urethra. In men, it lives inside the urethra. During sex without a condom, the parasite transfers through the exchange of genital fluids and direct tissue contact.
Vagina-to-penis and penis-to-vagina transmission are the most common routes. Vagina-to-vagina transmission also occurs. The parasite has not been shown to reliably spread through oral or anal sex, and it does not infect the mouth, throat, or rectum.
Can You Get It Without Having Sex?
Non-sexual transmission is technically possible but rare. The parasite can survive on damp surfaces for limited periods: up to 45 minutes on a toilet seat and up to 25 hours on a damp towel. These findings come from lab conditions, and documented cases of transmission through shared towels, washcloths, or wet surfaces are uncommon. The parasite is fragile and dies quickly once it dries out, so casual contact like hugging, sharing food, or sitting on a dry surface carries no meaningful risk.
Women who are infected during pregnancy can pass the parasite to their baby during vaginal delivery, though this is also uncommon.
Why So Many People Spread It Without Knowing
The biggest reason trichomoniasis spreads so easily is that most people who carry it have no idea they’re infected. Between 70% and 85% of people with trichomoniasis have minimal or no symptoms. In men, the vast majority of infections are completely silent, though some experience discharge from the penis or pain while urinating. Among women, roughly half develop vaginal discharge, but the other half may notice nothing unusual.
When symptoms do appear, they typically show up 5 to 28 days after exposure. But because so many infections stay quiet, a person can carry and transmit the parasite for months or years without a single noticeable sign. This is why someone can test positive for trichomoniasis even in a long-term, otherwise stable relationship. One partner may have acquired the infection before the relationship began and unknowingly carried it since.
What Happens Inside the Body
Once the parasite reaches the genital tract, it attaches to the surface cells lining the vagina or urethra. Research has shown that strains with stronger attachment ability establish more persistent infections, meaning the parasite’s grip on your tissue directly determines how well it takes hold. The parasite also releases tiny packages of molecules called extracellular vesicles that help it colonize more effectively, essentially preparing the surrounding tissue for a stronger foothold.
This attachment triggers inflammation, which is what causes symptoms when they do occur: irritation, redness, burning during urination, and changes in vaginal discharge. Left untreated, the infection does not resolve on its own. It persists indefinitely.
Risk Factors That Increase Your Chances
Certain behaviors and circumstances make acquiring trichomoniasis more likely:
- Unprotected sex: Condoms significantly reduce transmission risk, and their absence is the single biggest modifiable risk factor.
- Multiple sexual partners: More partners means more potential exposure to an asymptomatic carrier.
- Previous STIs: A history of other sexually transmitted infections correlates with higher trichomoniasis rates.
- Past trichomoniasis infection: Having been treated before does not create immunity. Reinfection is common, particularly if a partner was not treated at the same time.
- Douching: Washing or rinsing the vagina with water or other fluids disrupts the natural microbial balance, making the tissue more vulnerable to colonization.
Reinfection After Treatment
Trichomoniasis is curable with prescription medication, but getting treated once does not protect you from getting it again. Reinfection rates are high, particularly within the first three months after treatment. The most common reason is that a sexual partner was never tested or treated, so the parasite simply passes back during the next unprotected encounter. Both partners need treatment at the same time for the infection to be fully eliminated from the relationship. Retesting about three months after treatment is standard practice to catch reinfections early.