Sex has many hidden dangers. If you hadn’t already, you can add a parasitic infection to that list.

Penises and vaginas can pass along a parasite called Trichomonas vaginalis during contact that causes a common sexually transmitted disease called trichomoniasis. Don’t let the name “vaginalis” fool you — this infection affects both men and women, specifically about 3.7 million of them in the United States. Unfortunately, the Centers for Disease Control and Prevention say that “although symptoms of the disease vary, most women and men who have the parasite cannot tell they are infected.” That’s about 70 percent of cases, and Planned Parenthood says men are more likely to not experience symptoms.

Read: New Tool Fights HIV In Vagina

According to that organization, when women do experience symptoms, they could include a frothy and unpleasant smelling discharge from the vagina that may contain blood. They may also experience swelling in the groin, itchiness in and around the vagina and frequent urge to urinate, with the urine sometimes burning.

The CDC adds that the symptoms may come and go, and they can make sex feel unpleasant.

For women who are pregnant, there is a risk to the baby. The CDC explains that expectant mothers who develop the STD are more likely to give birth prematurely, and the babies are more likely to weigh less than 5.5 pounds. Because premature babies have not had as much time to develop in the womb, they can have all sorts of health problems, according to charity organization March of Dimes. Those include trouble breathing; jaundice; the blood disorder anemia; a weakened immune system that leaves them more susceptible to infection; bleeding in the brain; and heart problems. The earlier the baby is born, the greater the risk of these complications.

The good news is trichomoniasis can be cured with antibiotics. Simply ignoring it won’t do anything — the CDC says untreated trich infections can last months or years. And during that time, the STD will make you more likely to catch another: “Trichomoniasis can cause genital inflammation that makes it easier to get infected with the HIV virus, or to pass the HIV virus on to a sex partner.”

See also:

10 Deadly Sex Stories

What Flesh-eating Bacteria Looks Like