Sexually transmitted infections (STIs): an overview

Sexually transmitted infections (STIs), once called sexually transmitted diseases (STDs), are infections caused by bacteria, viruses or parasites that pass from one person to another mainly through vaginal, anal or oral sex. They are very common, often cause no symptoms, and almost all can be prevented, detected and treated.

STI or STD: why the name changed

Today STI is the preferred term. Many of these infections cause no symptoms or visible illness, but they can still be passed on. Saying "infection" is more accurate: it reminds us that you can have an STI and pass it on without knowing. It is the term used by the World Health Organization (WHO), the European Centre for Disease Prevention and Control (ECDC) and the NHS. "STD" still appears in searches and older texts and means the same thing.

How common STIs are

According to WHO, more than one million curable STIs are acquired every day worldwide among people aged 15 to 49. In 2020 there were around 374 million new cases of chlamydia, gonorrhoea, syphilis or trichomoniasis. In Europe, the ECDC reported record figures for 2024: 213,443 chlamydia cases, 106,331 gonorrhoea cases (up 303% since 2015) and 45,577 syphilis cases.

The most common STIs

WHO identifies eight organisms behind most STIs. Four are curable with treatment:

• Chlamydia: the most commonly reported bacterial STI in Europe.
• Gonorrhoea: a concern because of growing antibiotic resistance.
• Syphilis: rising, and can pass to a baby during pregnancy.
• Trichomoniasis: caused by a parasite; the most common curable STI worldwide.

The other four are viral. They cannot be cured, but there is treatment or a vaccine:

• HIV: with treatment, people living with HIV can lead long, healthy lives and, with an undetectable viral load, do not pass it on through sex.
• Human papillomavirus (HPV): very common; some types cause warts and others cervical cancer.
• Genital herpes: around 520 million people aged 15 to 49 carry the virus that most often causes it.
• Hepatitis B: prevented by a highly effective vaccine.

Vaginal thrush is often mistaken for an STI, but it isn't one: it is caused by a yeast that normally lives in the body.

How they are passed on

Through contact with genital fluids, blood, or skin and mucous membranes during vaginal, anal or oral sex, and by sharing sex toys without washing or covering them. Some, such as syphilis, HIV and hepatitis B, can also pass through blood or from mother to baby. They are not passed on by hugging, kissing on the cheek, toilet seats or swimming pools.

Prevention

• Condoms, external or internal, from start to finish. WHO says they are among the most effective methods, though they protect less against infections that cause sores outside the covered area, such as herpes or syphilis. Avoid the most common condom mistakes.
• Vaccines against HPV and hepatitis B.
• HIV prevention medicines: PrEP (before exposure) and PEP (after exposure, within 72 hours) are available from sexual health services.
• Testing and treating partners to avoid reinfection.

Testing: when to get tested

Tests are simple (urine, blood or a swab) and confidential. The US Centers for Disease Control and Prevention (CDC) recommends, for example, an HIV test at least once between ages 13 and 64, and yearly chlamydia and gonorrhoea tests for sexually active women under 25. Get tested too if you have a new partner, if a partner has an STI or if you notice symptoms. See how often to get tested for STIs.

Signs to see a doctor

Contact your GP or a sexual health clinic if you notice unusual discharge, pain when peeing, sores, warts or lumps on the genitals or anus, lower tummy pain, or bleeding between periods or after sex. Don't self-treat, and avoid sex until you have a diagnosis.