Can you get an STI in a relationship even if no one’s cheated?

Yes. A positive test doesn’t prove that anyone has cheated. The most common reasons are:

• one of you had an infection from a past partner that caused no symptoms
• symptoms appeared months or years after the infection was caught
• treatment didn’t clear the infection, or it was passed back and forth between you
• an earlier test didn’t check for that infection, or didn’t test the right part of the body
• the infection was passed on another way, such as through blood or skin contact

Some infections that cause genital symptoms, such as thrush and bacterial vaginosis, are not STIs at all.

Many STIs cause no symptoms, so it is possible to have one without knowing and pass it on to a new partner.

• Chlamydia: UK health officials estimate that around 70% to 80% of people with Chlamydia don’t know they have it.
• Gonorrhoea: most men with Gonorrhoea in the urethra get symptoms, but many women don’t, and infections in the throat and rectum usually cause no symptoms.
• HPV usually causes no symptoms.
• Herpes: most Herpes infections cause no symptoms or aren’t recognised, so many people pass it on without knowing.
• Trichomonas: many people get no symptoms at all.
• Mycoplasma genitalium: most people who have it don’t develop any illness.

Some infections can stay in the body for a long time before anyone notices them:

• Untreated Chlamydia can last for months. Only up to half of infections clear up on their own within about a year.
• People with HIV may have no symptoms for years.
• HPV is usually cleared within 2 years, but the NHS says you can have it even if you haven’t had a new partner for many years. HPV can stay in the body for a long time without being picked up, and it isn’t possible to work out when you got it. A positive HPV test doesn’t mean either partner has been unfaithful.
• Syphilis can enter a hidden stage with no symptoms and stay in the body for years without treatment.
• Herpes symptoms might not appear for weeks or even years after infection.
• Trichomonas can last for months or even years without treatment.

Nobody knows how long Mycoplasma genitalium takes to cause symptoms, and most people never get any.

Sometimes an infection is treated but comes back. This can happen when:

• a partner wasn’t treated at the same time, so the infection is passed back
• the course of treatment wasn’t finished
• the bacteria are resistant to the antibiotic. In England, 29 cases of Gonorrhoea resistant to ceftriaxone, the main treatment, were reported in 2025, more than double the 13 reported in 2024, and 21 more had been reported in 2026 by the end of August. Resistance is also common in Mycoplasma genitalium.
• the infection was in the throat. UK guidance says Gonorrhoea in the throat is harder to treat than in the genitals, so a test of cure is advised.

To avoid passing an infection back and forth, you and your partner should both be treated. Don’t have sex, including oral sex, until you have both finished treatment and been told it is safe to have sex again.

Not every STI test checks for every infection. A standard screen usually covers Chlamydia, Gonorrhoea, HIV and Syphilis. UK guidance doesn’t recommend routinely testing people without symptoms for Mycoplasma genitalium, Herpes or Trichomonas, so these can be missed.

Where the sample comes from matters too. A urine test won’t pick up Chlamydia or Gonorrhoea in the throat or rectum, so if you have had oral or anal sex, you may need a swab from those areas.

This means you or your partner may have tested negative in the past and still had an infection that the test didn’t look for.

Most STIs are passed on through vaginal, anal or oral sex, but some can also spread in other ways:

• Hepatitis B and Hepatitis C can spread through shared needles, razors and toothbrushes. Hepatitis C can also spread through tattoos done in unregulated places.
• HPV can spread through genital touching and any skin-to-skin contact of the genital area.
• Herpes can spread when a cold sore touches the genitals, or on fingers.
• Gonorrhoea in the throat may spread through deep kissing, according to research among gay and bisexual men.
• HIV can spread through shared needles. The risk from a blood transfusion in the UK is very low.

Read more in “7 ways you can catch an STI without having sex”

Some common genital infections are not STIs, even though they can cause similar symptoms.

Thrush is not an STI. The NHS says it can be triggered by sex or, less often, passed on during sex. Other triggers include antibiotics, pregnancy and poorly controlled diabetes.

Bacterial vaginosis (BV) is not an STI either, although sex can trigger it. It has even been found in women who have never had sex.

If you or your partner tests positive, try not to jump to conclusions. Talk about it calmly, and remember that the infection may have been there since before you met.

• Both of you should get tested, even if only one of you has symptoms.
• Get treated at the same time, and don’t have sex until you have both finished treatment and been told it is safe to have sex again.
• Ask whether you need a test of cure. This is recommended in some cases, such as Gonorrhoea in the throat.

The Better2Know Platinum Screen tests for 12 infections, including Chlamydia, Gonorrhoea, HIV, Syphilis, Trichomonas, Mycoplasma, Herpes 1 and Herpes 2, from 28 days after possible exposure. You can book it at one of our clinics or order a Platinum Screen home test.

For tips on the conversation, read “Positive STI test result? How to tell your partner”

Can you catch Chlamydia without cheating?

Yes. Most people with Chlamydia have no symptoms, so it can go unnoticed for a long time. Read more in “Can you catch Chlamydia without cheating?”

How long can you have an STI without knowing?

It depends on the infection. HIV, Syphilis, Herpes and HPV can all go unnoticed for years, and untreated Chlamydia and Trichomonas can last for months or longer.

Should couples get tested together?

Yes. Testing together when you start a relationship, or before you stop using condoms, means you both know where you stand. Read more in “Should we get tested before having sex? How to talk to a new partner”

You can get an STI in a relationship without anyone cheating. Many STIs cause no symptoms for months or years, treatment doesn’t always work first time, and some tests don’t check for every infection.

If you or your partner has symptoms or a positive result, get tested together. Book a Platinum Screen at a clinic near you or order a home test kit. Our Patient Care Team is available 24/7 on 020 7099 0955.

Not in the UK? Choose your country from the selector at the top of the page for local tests and prices.

This article originally appeared on 15 March 2024 and was last updated on 29 September 2026.

Medical review. This article was medically reviewed by Dr Steve Chapman, Doctor with a wealth of clinical experience in the field of STIs, on 3 February 2026. It is general information and not a substitute for personal medical advice.

Sources

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