Incubation period
5 to 28 days.
When a test becomes reliable
About 1 to 4 weeks after exposure.
Symptoms
- Around 7 in 10 people have no symptoms at all
- Frothy discharge, often yellow-green, with an unusual smell
- Itching, soreness or redness around the genitals
- Burning when passing urine, or discomfort during sex
- In men it is nearly always silent, which is how it keeps circulating
Good to know
- It is a single-celled parasite that swims using whip-like tails, not a bacterium and not a virus
- Around 156 million new infections a year, making it the most common curable STI worldwide
- It is the one on this list you can actually watch moving under a microscope
- Because it is so often silent in men, partners are usually found only after someone else is diagnosed
How it was discovered
Alfred Donné described it in Paris in 1836, looking at vaginal discharge under a microscope and seeing something swimming. It was one of the first human parasites ever observed alive, and for almost a century afterwards it was considered harmless — a curiosity that happened to live there. Only in the 1940s was it accepted as a cause of illness in its own right.
Sources:
Last updated: September 2026
This page is general information, not a diagnosis and not medical advice. Symptoms overlap between infections, and many infections have none at all — only a test can tell. If you think you have been exposed, talk to a doctor.