Get to Know Trichomoniasis, the Most Common Non-Viral Sexually Transmitted Infection

It was late one night in the lab when a physician from the emergency room came over. She had taken a couple of samples from a patient and wanted to read her own slides. We call it a wet mount when you read slides right after they’re collected. But this particular physician forgot one crucial step.

“Did you see anything in the slides?” I asked. I already knew the answer, but I wanted to see if she would catch on.

“Just a bunch of white blood cells,” she replied. “Nothing significant.”

I then took a saline vial with a dropper and put a drop of the saline on the slide, covered it, and took a look. “Have a look,” I said. She leaned into the microscope and said she didn’t see anything remarkable. “One sec,” I said, and then I tapped the table slightly.

The look on her face was priceless.

Trichomoniasis is one of the most common sexually transmitted infections in the world, yet many people have never heard of it. As someone who studies infectious diseases, I often find it concerning when an infection that affects millions of people doesn’t get the public attention it deserves. That lack of awareness means people are more likely to go undiagnosed and untreated, increasing the chances of serious health problems and continued spread. So let’s talk about what trichomoniasis is, who it affects, and why it matters.

Trichomoniasis (often shortened to “trich,” pronounced “trick”) is caused by a tiny parasite called Trichomonas vaginalis. Unlike viruses such as HIV or herpes, this is a non-viral infection, which means it’s caused by a living organism that isn’t a virus. The parasite infects the genitals and urinary tract: areas that include the vagina, cervix (the opening to the uterus), and vulva (external genitals) in women, and the urethra (the tube that carries urine) or head of the penis in men.

What makes trichomoniasis stand out is how widespread it is. Back in 2020, the World Health Organization estimated that around 156 million people between the ages of 15 and 49 got infected globally in just one year. That’s more than the total global cases of chlamydia, gonorrhea, and syphilis combined. In the United States, around 2.6 million people were infected in 2018. Most of them — about four out of five — were women, though men can also get infected and pass the infection to others. (This last point is important because men can be asymptomatic carriers of the disease, acting more like vectors.)

An Unequal Risk

One of the most important things to understand about trichomoniasis is that it doesn’t affect everyone equally. While anyone sexually active can get it, your chances can go up depending on several factors. Age plays a role, but not the way people often assume. Unlike infections like chlamydia, which are more common in younger people, trichomoniasis becomes more common with age. In one study, less than 1 out of 100 men under 30 tested positive, compared to more than 5 out of 100 in men over 30.

Sexual behavior matters, too. People with more than one sexual partner in a year tend to be at higher risk. But even your relationship status can influence your chances. One study from South Africa found that single people were more than twice as likely to have the infection compared to those in steady relationships. Though a complicating factor in that study was HIV status. And, as I hope you know, HIV already raises your risk for a plethora of diseases and conditions, especially infectious diseases.

What really stands out, though, is how deeply this infection is tied to racial and economic inequality. In the U.S., Black women are diagnosed with trichomoniasis at much higher rates than white women. While the national average for women is around 2%, it jumps to nearly 10% for Black women. Black men also face higher rates than their white counterparts. These differences aren’t caused by biology. They reflect structural inequities in healthcare access, income, education, and other social conditions.

To put this into perspective, imagine two fictional women living in the same city. One has private insurance, flexible work hours, and a nearby clinic that offers regular testing. The other works two jobs with no paid time off, lacks health insurance, and lives in a neighborhood with few medical resources. If both of them get infected with trichomoniasis, the first woman has a much better chance of getting tested and treated. The second might not even realize she’s infected until complications show up, and she could unknowingly pass the infection to others.

That’s especially concerning because most people with trichomoniasis don’t have symptoms. Around 70% of people with the infection feel fine and don’t know they have it. This silent spread makes regular testing even more important.

Okay, So What Are the Symptoms?

When symptoms do show up, they can vary. A woman might notice an unusual discharge from the vagina that smells different or looks greenish or foamy. (For the biochemists among you, that foam results from the organism’s use of fermentation to survive in an anaerobic environment.) There could also be irritation, burning during sex or while urinating, or just general discomfort. A man, on the other hand, might feel pain when peeing, see a discharge from the penis, or notice some redness or swelling, though these symptoms tend to be milder and less frequent.

Let’s use a fictional couple as an example. John, a 35-year-old man, feels a slight burning when he pees but assumes it’s from not drinking enough water. His partner, Sarah, notices some discharge and irritation but blames it on hormonal changes. Neither of them gets tested. In reality, they both have trichomoniasis and may keep passing it back and forth without knowing.

Trichomoniasis Can Get Complicated

Left untreated, trichomoniasis can cause real health problems. For pregnant women, it increases the risk of having the baby too early. The infection also causes inflammation, which makes it easier to get or spread HIV (the virus that causes AIDS). There’s also growing research connecting trichomoniasis with other reproductive health issues, like pelvic inflammatory disease (an infection of the uterus, fallopian tubes, or ovaries).

The good news is that it’s easy to diagnose and treat. A healthcare provider can test for it with the wet prep procedure I described. Once diagnosed, the infection is usually treated with a single dose of metronidazole (an antibiotic that kills the parasite). It’s important that both sexual partners are treated at the same time to avoid passing the infection back and forth. Skipping partner treatment is one of the main reasons people get reinfected quickly.

How Do I Avoid It Altogether?

Preventing trichomoniasis is a lot like preventing other sexually transmitted infections. Using condoms correctly during sex helps reduce the risk. Regular testing also plays a big role, especially for people with new or multiple partners. Since many people don’t have symptoms, waiting until you feel something is wrong isn’t a reliable approach.

Going to regular checkups helps, because you could be carrying it and not know it. Getting treated prevents you from passing it on. But you also have to be honest with your healthcare provider and tell them about your sexual history. Are you with multiple partners? Do you suspect or know if a partner is with others, not just you? And are you concerned about other sexually transmitted infections?

Leave the taboos at the door when talking to a provider.

Once I tapped on the table, all the little Trichomonas parasites came alive. You see, the physician had let the slides dry. When that happens, the parasite reacts by going dormant. Even adding the saline may not be enough. When I added the saline and looked, I saw the parasites were still. I recognized them because that was my job.

However, the parasites look like white blood cells to the untrained eye. They’re about the same size and shape. And the flagella (little hair-like structures they use to move) can be too thin to be seen without a stain. The physician could see them once they were on the move, which is something white blood cells don’t do.

This photomicrograph of a wet-mounted vaginal discharge specimen, revealed numbers of Trichomonas vaginalis protozoan parasites, leading to a diagnosis of trichomoniasis, which is a very common sexually transmitted disease (STD) caused by the protozoan, T. vaginalis. Although symptoms of the disease vary, most women and men, who harbor the parasite, are unaware that they are infected. (Image via CDC’s Public Health Image Library. Public domain.)

So, doctors, please ask the lab techs for a consult when you go into the lab.

Trichomoniasis doesn’t often make headlines, but that doesn’t mean it isn’t important. It affects millions of people, yet many have never heard of it. That silence can lead to missed diagnoses, untreated infections, and long-term health problems. The more we talk about it, the more we can encourage testing, reduce stigma, and protect each other’s health.

If you’re sexually active and haven’t been tested recently, consider asking your healthcare provider about trichomoniasis along with other STI tests. Taking care of your sexual health is just as important as caring for the rest of your body, and it starts with staying informed.