Trichomoniasis is not a bacterial infection. It’s caused by a single-celled parasite called Trichomonas vaginalis, which makes it a protozoal infection. The confusion is understandable: trichomoniasis produces symptoms that overlap with bacterial vaginosis, it’s treated with a medication that also kills bacteria, and it’s grouped alongside bacterial STIs in most health resources. But the organism responsible is fundamentally different from bacteria.
Why It Gets Confused With a Bacterial Infection
Trichomonas vaginalis is a protozoan, the most common disease-causing protozoan in humans in industrialized countries. Under a microscope, it looks nothing like a bacterium. It’s pear-shaped, roughly 7 to 30 micrometers long, and has five whip-like tails called flagella that it uses to move around. Bacteria are far simpler organisms without these structures. Protozoa are larger, more complex single-celled organisms that behave more like tiny animals than like bacteria.
The main reason people assume trichomoniasis is bacterial comes down to two things. First, the symptoms can look a lot like bacterial vaginosis (BV), another common vaginal condition. Both can cause abnormal discharge and a fishy odor. Second, the go-to treatment for trichomoniasis is metronidazole, a drug most people know as an antibiotic. That name is technically accurate since metronidazole does kill certain bacteria, but it also destroys protozoan parasites through the same core mechanism: it enters the organism, gets chemically activated inside the cell, and damages the parasite’s DNA until the cell dies. The drug works against both types of organisms, which is why the FDA lists Trichomonas vaginalis alongside bacterial species as targets for metronidazole.
How It Differs From Bacterial Vaginosis
Trichomoniasis and BV are frequently mistaken for each other, but they have different causes and different implications. BV results from an imbalance in the natural bacteria living in the vagina, with certain bacterial species overgrowing. Trichomoniasis is a sexually transmitted infection caused by a parasite passed between partners during genital contact.
The discharge patterns differ subtly. BV typically produces a thin, white or gray discharge with a strong fishy smell, especially after sex. Trichomoniasis can cause a discharge that ranges from clear to white, sometimes with a yellowish or greenish tint, and may also carry a fishy odor. Trichomoniasis is more likely to cause itching, irritation, and discomfort during urination, while BV often has fewer inflammatory symptoms. That said, overlap is common enough that lab testing is the only reliable way to tell them apart.
Most People Don’t Know They Have It
One of the most striking things about trichomoniasis is how quietly it can persist. Between 70% and 85% of infected people have minimal or no symptoms at all. Without treatment, the infection can last months to years. The CDC estimates there were more than two million trichomoniasis infections in the United States in 2018, and many of those people likely had no idea they were carrying the parasite.
Men are particularly unlikely to notice symptoms. When symptoms do appear in women, they typically include vaginal irritation, burning during urination, and changes in discharge. In men, the infection can cause mild irritation inside the penis or a slight discharge, but it often clears on its own in men while persisting indefinitely in women without treatment.
Why Untreated Trichomoniasis Matters
Even without symptoms, an untreated trichomoniasis infection carries real health consequences. The parasite causes low-level inflammation in the genital tract, and that inflammation has downstream effects. Studies from multiple countries have found that trichomoniasis increases the risk of acquiring or transmitting HIV by roughly two to three times. A study following over 400 HIV-negative women over time found that prior trichomoniasis infection doubled the rate of HIV seroconversion, meaning the parasite meaningfully raises vulnerability to a second, more serious infection.
During pregnancy, untreated trichomoniasis is linked to low birth weight, preterm delivery, and premature rupture of membranes. In rare cases, the parasite can be transmitted to the baby during birth, potentially causing vaginal or respiratory infections in the newborn.
How It Spreads
Trichomoniasis spreads through genital-to-genital contact during sex. The parasite can be transmitted between a penis and a vagina, or between two vaginas. It does not typically infect the mouth or rectum. Both partners need treatment at the same time to prevent passing the infection back and forth, a pattern called “ping-pong” reinfection that is one of the most common reasons the infection persists.
Treatment Is Simple but Specific
Trichomoniasis is curable with prescription medication. Metronidazole or a closely related drug, tinidazole, are the standard options. These drugs belong to a class called nitroimidazoles, which work by entering the parasite’s cell and generating reactive molecules that destroy its DNA from the inside. The treatment is taken by mouth, not applied topically, and typically resolves the infection quickly.
The key detail many people miss is that both sexual partners must be treated simultaneously. If only one person takes the medication, the untreated partner will simply reinfect them. You should also avoid alcohol during and for a period after taking these medications, as the combination can cause nausea and vomiting. Retesting about three months after treatment is recommended to confirm the infection is gone, since reinfection rates are high.
The Bottom Line on Classification
Trichomoniasis sits in an odd category in most people’s minds. It’s sexually transmitted like chlamydia and gonorrhea, which are bacterial. It’s treated with a drug commonly called an antibiotic. And it produces symptoms that mimic bacterial vaginosis. But the organism itself is a protozoan parasite, not a bacterium, and that distinction matters for understanding how it behaves, how it’s tested for, and why both partners need treatment to break the cycle of reinfection.