Trichomoniasis, sometimes called trich, is an infection caused by a parasite. It’s one of the most common curable sexually transmitted infections (STI). About 3.7 million people in the United States have it.

In women, trichomoniasis can cause:

  • itching, burning, and redness in and around the vagina
  • painful urination
  • pain during sex
  • smelly yellow, green, or white discharge from the vagina
  • lower abdominal pain

In men, trichomoniasis can cause:

  • burning after ejaculation
  • white discharge from the penis
  • pain or burning during urination
  • swelling and redness around the head of the penis
  • pain during sex

The symptoms tend to show up anywhere from 5 to 28 days after you’ve been exposed to the parasite. Trichomoniasis is spread through sexual contact. So, how can you get trichomoniasis is no one cheats in a relationship? In very rare cases, it can spread through sharing personal items, such as towels.

Read on to learn more about how trichomoniasis spreads and whether it’s a sign that your partner is cheating.

Trichomoniasis is caused by a parasite called Trichomonas vaginalis that can live in semen or vaginal fluids. It spreads during unprotected anal, oral, or vaginal sex, usually between a man and a woman or between two women. Keep in mind that a man doesn’t have to ejaculate to give his partner the parasite. In can also be spread by sharing sex toys.

In men, the parasite usually infects the urethra inside the penis. In women, it can infect the:

  • vagina
  • vulva
  • cervix
  • urethra

If you’re in a committed relationship and your partner suddenly develops an STI, your mind probably immediately jumps to infidelity. While trichomoniasis is almost always spread through sexual contact, about 70 percent of people with the infection don’t show any symptoms.

People can also carry the parasite for many months without knowing it. This means that your partner may have gotten it from a past relationship and only just started showing symptoms. It also means that you might have developed an infection in a past relationship and unknowingly passed it to your current partner.

Still, there’s always a (very) slim chance that you or your partner developed it from something nonsexual, such as:

  • Toilets. Trichomoniasis can be picked up from a toilet seat if it’s damp. Using an outdoor toilet may be an added risk, since it puts you in closer contact with others’ urine and feces.
  • Shared baths. In one study from Zambia, the parasite spread through bathwater that was used by multiple girls.
  • Public pools. The parasite can spread if the water in the pool isn’t cleaned.
  • Clothing or towels. It’s possible to spread the parasite if you share damp clothing or towels with someone.

Keep in mind that there are very few reported cases of trichomoniasis being spread through these means, but it is possible.

If your partner tests positive for trichomoniasis or you have symptoms of it, see a healthcare provider to get tested. This is the only way to know if you have the infection. The Centers for Disease Control and Prevention have a tool that helps you find free STI testing in your area.

If you do test positive for trichomoniasis, you might also be tested for chlamydia or gonorrhea. People with trichomoniasis often have these STIs, too. Having trichomoniasis can also increase your risk of developing another STI, including HIV, in the future, so it’s important to follow up with treatment.

Trichomoniasis is easily treated with antibiotics, such as metronidazole (Flagyl) and tinidazole (Tindamax). Make sure you take the full course of antibiotics. You should also wait about a week after you finish your antibiotics before having sex again.

If your partner gave it to you, they’ll also need treatment to avoid reinfecting you.

People can have trichomoniasis for months without showing any symptoms. If you or your partner suddenly have symptoms or test positive for it, it doesn’t necessarily mean that someone’s cheating. Either partner may have gotten it in a previous relationship and unknowingly passed it on. While it’s tempting to jump to conclusions, try have an open, honest conversation with your partner about their sexual activity.