The most common non-viral STI is often entirely invisible

Science · Published 23 October 2026 · Updated 23 October 2026 Swipe more like this →

In short: While many sexually transmitted infections dominate the headlines, a microscopic protozoan parasite causes millions of new infections every year. Often presenting with no symptoms at all, Trichomonas vaginalis can spread silently, increasing the risk of serious health complications if left undetected and untreated.

Trichomoniasis is caused by the protozoan parasite Trichomonas vaginalis, which infects the urogenital tract. In 2020, there were an estimated 156 million new cases globally among people aged 15–49, including approximately 73.7 million females and 82.6 million males. One third of these new infections occurred in the WHO African Region, followed by the Americas.

The infection is frequently asymptomatic, making it difficult to track without testing. When symptoms do appear, they can manifest differently depending on biological sex. In women, the infection may cause a yellow-green, frothy, or purulent vaginal discharge that sometimes carries a fishy odor, alongside itching, soreness, or pain during urination and intercourse. In men, symptoms may include urethral discharge, penile irritation, or pain during ejaculation.

If left untreated, the parasite can lead to significant health complications. For women, it is associated with pelvic inflammatory disease (PID)—which can lead to infertility or ectopic pregnancy—and an increased risk of bacterial vaginosis or HIV acquisition. During pregnancy, the infection is linked to preterm birth, low birthweight, and an increased risk of sepsis for the mother after delivery.

Fortunately, the condition is both preventable and curable. Consistent and correct condom use is the most effective prevention method. Diagnosis can be achieved through molecular testing, such as nucleic acid amplification tests (NAAT), which are considered a gold standard. Treatment typically involves antibiotics, most commonly metronidazole, taken either as a single dose or twice daily for seven days. It is vital to complete the full course and ensure sexual partners are also treated to prevent reinfection.

General information, not professional advice. Spotted an error? Flag the card or see corrections and complaints.

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