Sexually transmitted infections

Overview

Sexually transmitted infections (STIs) are infections passed from person to person primarily through sexual contact. Common STIs include chlamydia, gonorrhoea, syphilis, herpes, human papillomavirus (HPV), HIV, and trichomoniasis. Many STIs cause no symptoms. Early testing and treatment prevent complications and onward transmission.

How common is it?

STIs are common, and many cause no symptoms. Frequency varies by infection, age, population and location; clinic diagnoses do not count every infection.

Causes and risk factors

Each STI has a specific pathogen. They are transmitted through vaginal, anal, or oral sex, and some through blood or skin-to-skin contact with infected areas.

Common risk factors

• Unprotected vaginal, anal, or oral sex
• Multiple sexual partners
• Young age (under 25 years)
• Not using condoms consistently
• Previous STIs (increases susceptibility)
• Sex work or partner who has multiple partners
• HIV infection (increases susceptibility to other STIs and vice versa)
• Some STIs transmissible perinatally or via blood (HIV, syphilis, hepatitis B)

Symptoms

• Many STIs are asymptomatic
• Unusual discharge from the penis, vagina, or rectum
• Burning or pain when urinating
• Sores, ulcers, or blisters on genitals or mouth (herpes, syphilis)
• Rash (syphilis secondary stage)
• Warts on or around the genitals or anus (HPV)
• Lower abdominal pain in women (pelvic inflammatory disease from chlamydia or gonorrhoea)
• Testicular pain and swelling
• Sore throat after oral sex

When to see a doctor

Contact a sexual health service for symptoms, a partner's diagnosis or concern after sexual exposure; testing frequency depends on your circumstances. Possible HIV exposure needs immediate assessment for PEP, ideally within 24 hours and no later than 72 hours. Do not wait for symptoms or test results.

Diagnosis

Testing varies by STI: urine or genital swab for chlamydia and gonorrhoea; blood test for HIV, syphilis, and hepatitis B and C; swab of sores for herpes; visual diagnosis or biopsy for HPV warts. Home testing kits are available for some STIs.

Treatments

Self-care and lifestyle

• Use condoms consistently and correctly with all new or non-tested partners
• Ask about HPV and hepatitis B vaccination under your local programme. In the UK, NHS HPV eligibility depends on age, birth cohort and risk group; it is not offered to every adult up to 45.
• Agree a testing schedule with a sexual health professional. If an STI needs treatment, avoid sex until you and partners have completed treatment and the clinic says it is safe.
• Discuss PrEP with a sexual health clinic if at high risk of HIV

Prevention

Consistent condom use, regular testing, HPV and hepatitis B vaccination, and PrEP for HIV prevention are the cornerstones. Limiting the number of sexual partners reduces exposure risk.