Can an STI lower my sperm count?

By spreading to the testicles, but it’s rare. The STIs most likely to matter are Chlamydia and Gonorrhoea. If they aren’t treated, they can spread from the urethra, the tube that carries pee and semen, to the testicle and to the epididymis, the tube behind each testicle that carries sperm. Doctors call this epididymo-orchitis.

In men under 35, an STI is the usual cause of this kind of infection. If it isn’t treated, it can occasionally cause lasting damage. UK patient information says that, very rarely, it leads to shrinkage of the testicle and loss of fertility.

How rare is it? UK guidelines estimate that epididymitis affects about 2% of men with Chlamydia who have no symptoms. The World Health Organization (WHO) says that in rare cases this can lead to infertility. UK guidelines don’t give a figure for how many men become infertile because of an STI.

Symptoms of epididymitis can include:

• sudden or gradual pain in one or both testicles
• a scrotum that feels tender, warm and swollen
• a build-up of fluid around the testicle that feels like a lump or swelling

You may also find it hard to pee, or notice a white, yellow or green discharge from the tip of your penis. See a doctor as soon as you can if you have any of these symptoms. Epididymitis is usually treated with antibiotics. You should start to feel better within a few days, but it can take up to 3 months to recover fully.

Call 999 or get urgent medical care straight away if you have sudden, severe pain in a testicle, testicle pain that has lasted more than an hour, or testicle pain along with feeling sick, being sick or pain in your tummy. It could be a twisted testicle, which needs to be treated in hospital as soon as possible. Read more in “What STIs cause epididymitis?”

The research is mixed, and much of it comes from small studies of men attending fertility clinics:

• several studies have linked Chlamydia in semen or urine with lower sperm numbers or poorer sperm function
• others have found no difference, and a 2013 review called the results inconclusive
• European urology guidelines say a review of the evidence couldn’t draw a strong link between STIs and male infertility, because the quality of the data was limited
• few studies have looked at Gonorrhoea and sperm quality

The official view is more reassuring. The UK Health Security Agency says complications of Chlamydia are much rarer in men than in women. UK guidelines say pain in the testicles is rare with Gonorrhoea, and that the link between infection in the testicles and fertility is poorly understood.

For men, lasting harm to fertility from Chlamydia or Gonorrhoea is the exception. Read more in “Can Chlamydia make you infertile?”

Herpes

UK guidance doesn’t list infertility as a complication of Herpes. Some studies have found the Herpes virus in the semen of men with fertility problems, but a 2013 review found the results on sperm count and movement very contradictory.

HPV

In a 2017 analysis of 31 studies, HPV was found in the semen of about 20% of men attending fertility clinics, compared with about 11% of men in the general population. Only 4 of those studies tested the link with infertility directly. They found that infertility was more likely in men with HPV in their semen, but European urology guidelines say more research is needed.

Mycoplasma genitalium

Mycoplasma genitalium can occasionally spread to the testicles. UK patient information says there’s no evidence that it causes male infertility.

HIV

Studies from before modern treatment found lower semen quality in men with HIV. UK guidelines say the effect of HIV on fertility now looks smaller than once thought.

Syphilis

The NHS lists the testicles among the organs that untreated Syphilis can damage many years later. It’s rare to reach that late stage in the UK.

Often, but not always. A 2012 review found that most men with epididymitis have a temporary drop in semen quality while they’re infected, but that lasting effects aren’t uncommon, even after the infection has been cured.

Early treatment gives you the best chance. UK patient information says prompt and early treatment significantly reduces the likelihood of complications.

If the epididymis is blocked and this is causing infertility, the NHS says surgery can be used to correct the blockage. Sperm can sometimes be collected surgically for fertility treatment too.

An infection is only one of many possible causes. The NHS says many cases of abnormal semen are unexplained. Known causes include:

• damage to the testicles from an infection, an injury, surgery or testicular cancer
• a testicle that hasn’t descended into the scrotum
• low testosterone
• some medicines, including chemotherapy and anabolic steroids
• illegal drugs, such as marijuana and cocaine

Smoking can reduce semen quality too. The NHS also suggests drinking less alcohol, losing weight if you’re overweight and wearing loose-fitting underwear.

Sperm problems are common. For around half of heterosexual couples who have trouble conceiving, the cause is sperm related. Read more in “Which STIs can cause infertility?”

With a semen analysis. A lab checks a sample of your semen for:

• the amount of sperm in it
• how fast the sperm move
• the shape of the sperm

WHO’s reference figures put the lower limit for sperm count at 16 million sperm per millilitre of semen. A result below this doesn’t prove you’re infertile, and the NHS says it may still be possible to conceive naturally with a low sperm count. If the first test finds a possible problem, the NHS repeats it around 3 months later.

See a doctor if you’ve been trying for a baby for a year without success. Go sooner if your partner is 36 or over, or if you think you might have had an STI.

Better2Know offers:

• Chlamydia and Gonorrhoea (Urine): tests one urine sample for both infections, from 14 days after sex, or straight away if you have symptoms, with results in 2 days. You can book it at one of our clinics or have a nurse visit you, and there’s also a home test kit.
• Full Screen: tests for 7 infections, including Chlamydia, Gonorrhoea, Syphilis and HIV, from 28 days after sex, with results in 3 days (5 days for the home test).
• Sperm Count: a clinic test that counts the sperm in a semen sample and assesses how healthy they are. If your count is below the normal range, we’ll arrange for you to speak to one of our doctors.

If you test before 45 days, a negative HIV result needs a repeat test. A Syphilis test in the first 3 months after sex can miss the infection, so you may need a repeat test. Not sure which test you need? Our Patient Care Team is available 24/7 on 020 7099 0955.

Can Chlamydia make a man infertile?

Rarely. Chlamydia can spread to the testicles, and WHO says this leads to infertility in rare cases. Complications of Chlamydia are much more common in women.

Will treating an STI improve my sperm count?

It may. Semen quality often drops during an infection in the testicles, and for many men the drop is temporary. Early treatment lowers the chance of lasting problems.

Can a sperm count test show whether I’ve had an STI?

No. A sperm count test measures sperm, not infections. You need an STI test to find out whether you have an infection.

An STI can lower your sperm count, but it’s rare, and it usually only happens when an untreated infection spreads to the testicles. Chlamydia often causes no symptoms, and not everyone with Gonorrhoea gets them, so a test is the only way to find them early. Early treatment helps protect your fertility, and your partner’s too.

Book a Chlamydia and Gonorrhoea test 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 27 October 2023 and was last updated on 3 October 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 20 July 2026. It is general information and not a substitute for personal medical advice.

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