An STI can be passed from one person to another through sexual contact, including vaginal, anal and oral sex. You can get or pass on an STI whoever you're having sex with.
STIs can pass between men and women, and from women to women and men to men. For more specific sexual health advice, read women who have sex with women and sexual health for men who have sex with men.
Many STIs can be cured with antibiotics. Some, such as HIV, have no cure, but can be treated to prevent them getting worse.
You can't tell by looking at someone (including yourself) whether they've got an infection, so it's important to get a check-up if you've had unprotected sex or think you might be at risk.
Many people don't notice symptoms when they have an STI, including most women with chlamydia. If it's left untreated, chlamydia can affect your ability to get pregnant.
Gonorrhoea can also affect fertility. Around 50% of women and 10% of men with gonorrhoea don't have symptoms.
Left untreated, STIs can affect your health. If you have any of the symptoms listed below, get tested.
In women and men:
- pain when you pass urine (pee)
- itching, burning or tingling around the genitals
- blisters, sores, spots or lumps around the genitals or anus
- black powder or tiny white dots in your underwear - this could be droppings or eggs from pubic lice
- yellow or green vaginal discharge
- discharge that smells
- bleeding between periods or after sex
- pain during sex
- lower abdominal pain
- discharge from the penis
- irritation of the urethra (the tube urine comes out of)
These symptoms don't necessarily mean you have an STI, but it's worth seeing a doctor so you can find out what's causing the symptoms and get treatment.
For example, it's possible to get thrush without having sex, but it can cause STI-like symptoms, such as soreness, itching and discharge. Thrush is easily treated - find out more about treatment for thrush.
Where can I get tested for STIs?
You can get tested at:
- a sexual health clinic or genitourinary medicine (GUM) clinic - find STI services near you
- some community contraceptive clinics - find contraceptive services near you
- some sexual health services - call the national sexual health line on 0300 123 7123 or Worth Talking About (for under-18s) on 0300 123 2930
- some GP surgeries
Some pharmacies can also test for chlamydia.
Find out where you can get a free chlamydia test through the National Chlamydia Screening Programme (under-25s only).
Have safer sex
Always use condoms to help protect yourself from catching or passing on an STI. Buy condoms that have the CE mark or BSI kite mark on the packet.
This means they've been tested to high safety standards. Condoms that don't have the CE mark or BSI kite mark won't meet these standards, so don't use them.
Find out what happens when you visit a sexual health clinic.
Article provided by NHS Choices