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Trichomonas

The commonest STI you have never heard of, caused by a parasite you can watch swimming. Underestimated, and not harmless.

SR
Dr Seth Rankin
MBChB MRCGP, Founder
1 July 2026 4 min read

Every other infection in this cluster is a bacterium or a virus. Trichomonas is neither. It is a parasite, a single-celled protozoan that propels itself with little whip-like tails, and it is the only common STI you can literally watch swimming across a microscope slide. That novelty is about the only charming thing about it, and it is one of the most common curable STIs in the world while remaining almost unknown to the public.

Why it is not just a curiosity

Trichomonas has a nasty habit of being dismissed. In women it can cause discharge, soreness, itching and discomfort, though it is often quiet; in men it usually causes nothing at all, which makes men efficient, oblivious carriers. Left untreated it is not benign. It inflames the genital tract, and inflamed tissue is easier for HIV to enter, so trichomonas measurably increases the risk of both catching and passing on HIV. In pregnancy it is linked to premature birth and low birth weight. So the infection filed under harmless and embarrassing is in fact a quiet amplifier of more serious things.

How it is found and fixed

It is found from a swab or a urine sample, and it is worth saying that it is not always in a basic panel, so if it is relevant to you a doctor makes sure it is actually tested for rather than assumed absent. Treatment is usually a short course of the antibiotic metronidazole, with the traditional warning that alcohol during the course is a memorably bad idea. It responds well most of the time, but it can be stubborn, and persistent infections that shrug off the first course, or that keep coming back because a partner was never treated, need higher doses or repeat courses. Partners are treated at the same time, because otherwise the pair of you simply hand it back and forth indefinitely, which is nobody's idea of a shared hobby.

The honest summary

Trichomonas is common, often silent, frequently overlooked, and quietly raises the stakes on HIV and pregnancy. Caught and treated properly, with the partner treated too, it usually clears. Left circulating between untreated partners, it becomes the infection that never quite goes away.

SR
Clinically reviewed
Dr Seth Rankin
MBChB MRCGP, Founder, LoveMyLife

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