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How do STIs affect your immune system?

07 Feb 2024 • 2 min read

Even the thought of a sexually transmitted infection (STI) can leave you feeling itchy. But they're more than just uncomfortable – they can make you ill and affect the way your body reacts to infection in the future.

How common are STIs?

You might not hear much about them, but STIs are more common than you might think. Almost 400,000 were reported in England in 2022.1 

Chlamydia makes up half of all reported STIs. Then it’s gonorrhoea, at just over 20%.1

Do STIs affect your immunity?

Some STIs don’t show symptoms for a while, or even at all. But that’s not to say they’re harmless.2

They may increase your likelihood of contracting other STIs, including HIV, in the future. And your individual genetic makeup might influence your risk of infection, too.3

Left untreated, certain STIs may impact your fertility, your brain health, and more.2,4 One area you might not know about is your immune system...

Human immunodeficiency virus (HIV)

Without treatment, HIV damages your body’s immunity and eventually leads to acquired immunodeficiency syndrome (AIDS). AIDS can leave you severely immunocompromised and at risk of life-threatening illnesses.5

Today, most people can expect to live a near-normal lifespan with HIV.5 It’s possible for HIV to become undetectable with the help of medications, which means that if the infection is low enough, for long enough, it won’t be detected in tests and cannot be passed on to others.5


Certain STIs have ways to evade your body’s usual immune response. Chlamydia enters via your epithelial cells, bypassing the antibodies which help protect you.3

Once the body recognises the infection, it has an extreme immune response to try and get rid of it. However, this can cause other conditions, like pelvic inflammatory disease (PID).3 Most people won’t know they have chlamydia unless they get tested, but symptoms can include:
  • Pain when peeing 
  • Unusual discharge from the vagina, penis, or bottom 
  • In women, stomach pain and bleeding after sex or in between periods 
  • In men, pain and swelling in the testicles


Like chlamydia, gonorrhoea can bypass your normal immune response and damage the cells that help protect you.3 By the time your body launches an attack, you may already be infected.

Symptoms include thick green or yellow discharge from your penis or vagina, pain when peeing, and bleeding in between periods.7


Genital warts are caused by the human papillomavirus (HPV), which is different to HIV. It’s possible to have HPV without warts.8

Most sexually active people will get HPV at some point in their lives. Usually, it’s harmless: the immune system fights it off and it goes away. But other times your body can’t get rid of the infection and you’ll need to see a doctor.9

Some types of HPV can cause abnormal cells in some people, and these abnormal cells could turn into cancer.8 That’s why it’s important to get tested regularly, and to keep up to date with your cervical screenings (smear tests) if you’re eligible.

Nowadays, schoolchildren aged 12 to 13 are offered an HPV vaccine. Ask your GP if you’re unsure whether you’ve had it. UK patients can get one for free if you’ve missed it and you’re under 25, or you’re otherwise considered at high risk.10,11


Syphilis has a reputation for being an older disease, but there were over 8600 cases in England in 2022.1

It can cause potentially life-threatening complications if not treated, so it’s vital to get tested if you think you may have it. Symptoms include:
  • Sores on your penis, vagina, or around your anus 
  • Sores on other areas of your body 
  • White or grey wart-like growths around your genitals and bottom 
  • White patches in your mouth 
  • A non-itchy rash on your hands, which may spread around your body 
  • Flu-like symptoms 
  • Patchy hair loss
It may also affect your immune system. There’s evidence that people who have both HIV and syphilis experience a decrease in CD4 cells, the “helper” cells that fight illness.12

Genital herpes

Herpes causes small blisters around the genitals. They clear up on their own but can come back again.13

You might have heard that herpes comes from the same virus as cold sores. This is true, although they’re two different strains (HSV-1 and HSV-2).14

Having a cold sore doesn’t mean you’ll also get genital herpes. However, you can catch genital herpes if a cold sore touches your genitals.14,13 If you’ve had herpes before, you’re more likely to get a recurrence when your immune system is low.13


This is caused by a parasite called Trichomonas vaginalis.15 If not treated, it damages the vaginal barrier and roughly doubles a woman’s risk of getting HIV.16 There may also be a link between the parasite, infertility, and prostate cancer in men.16 Look out for: 15 
  • Abnormal or fishy-smelling vaginal discharge 
  • Soreness, swelling, and itching around the vagina 
  • Pain while peeing or ejaculating 
  • Thin, white discharge from the penis 
  • Swelling and soreness around the head of the penis or foreskin

How can I avoid STIs?

It’s impossible to completely avoid STIs, but you can help lower your risk by using a condom and getting tested regularly.
If you’re: 
  • A woman or person with a womb or ovaries 
  • Aged under 25 
  • Sexually active
You should be screened for chlamydia with every new sexual partner or once a year, whichever comes first. You should be screened for all STIs once a year.17

If you’re a man who’s sexually active with other men, you should test for HIV and STIs every year, or every 3 months if you’re having sex without a condom with new partners.17

Don’t wait if you think you might have been exposed to an STI, though.17 You can also get vaccinated against some STIs, like HPV and hepatitis B.2

The final say

Anyone who’s sexually active can get an STI, no matter your age or how often you have sex. Having one isn’t dirty and is nothing to be embarrassed about. But you’ll need to get medical help as fast as possible to keep yourself and any partners healthy.

Handpicked content: Why is sex painful?
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