Sexually Transmitted Infections — Symptoms, Causes & Treatment | MyMedicPlus

Sexually transmitted infections (STIs) encompass over 30 bacterial, viral, and parasitic pathogens primarily transmitted through sexual contact — including vaginal, anal, and oral sex. The WHO estimates over 1 million new curable STIs are acquired daily worldwide, with 374 million new infections of the four most prevalent curable STIs (chlamydia, gonorrhoea, syphilis, and trichomoniasis) occurring annually. Major bacterial STIs include chlamydia (Chlamydia trachomatis — most common bacterial STI globally), gonorrhoea (Neisseria gonorrhoeae — rising antibiotic resistance), and syphilis (Treponema pallidum). Major viral STIs include HIV (human immunodeficiency virus — 38 million people living with HIV globally), HPV (most prevalent STI worldwide — over 100 strains), herpes simplex virus (HSV-1 and HSV-2 — affecting approximately 3.7 billion people under 50 globally), and hepatitis B. Parasitic STIs include Trichomonas vaginalis and pubic lice. Many STIs are curable with antibiotics when detected early; viral STIs are generally manageable but not curable (with the exception of hepatitis C). Screening and partner notification are essential because the majority of common STIs — including chlamydia and HIV — are frequently asymptomatic.

Bacterial STIs: Chlamydia trachomatis, Neisseria gonorrhoeae, Treponema pallidum (syphilis), Mycoplasma genitalium, Haemophilus ducreyi (chancroid). Viral STIs: HIV, HSV-1 and HSV-2, HPV (over 100 types — high-risk types 16 and 18 cause cervical and oropharyngeal cancers; low-risk types 6 and 11 cause genital warts), hepatitis B (HBV — sexually transmitted and blood-borne). Parasitic: Trichomonas vaginalis, Phthirus pubis (pubic lice), Sarcoptes scabiei. Transmission routes: vaginal, anal, or oral sexual contact; blood-to-blood (HIV, HBV, HCV); mother-to-child during pregnancy, labour, or breastfeeding. Risk factors: unprotected (condomless) sex, multiple concurrent sexual partners, men who have sex with men (MSM), intravenous drug use (sharing needles), young age 15–24 years (highest chlamydia incidence), history of prior STI (biological susceptibility increased), sex work, alcohol and substance use (impairs risk decision-making), and existing STI (genital ulcers significantly increase HIV acquisition and transmission risk).

Many STIs are asymptomatic, particularly chlamydia (70% of women, 50% of men have no symptoms), gonorrhoea (in women), and early HIV and syphilis — which is why regular screening is essential regardless of symptoms. When symptoms occur: Urethral discharge: purulent yellow-green discharge in gonorrhoea; clear or white in chlamydia; dysuria (pain on urination). Vaginal symptoms: increased or offensive discharge (Trichomonas vaginalis — frothy yellow-green; bacterial vaginosis — grey with fishy odour); intermenstrual or post-coital bleeding (chlamydia cervicitis). Genital ulcers: painful, multiple small vesicles progressing to ulcers (herpes simplex — primary episode with fever, lymphadenopathy); painless indurated single chancre at inoculation site (primary syphilis — resolves spontaneously without treatment but progresses internally). Secondary syphilis: disseminated rash (typically involving the palms and soles — pathognomonic), condylomata lata, alopecia, and constitutional symptoms. Genital warts: flesh-coloured cauliflower-like papules on genitalia and perianal area from HPV types 6 and 11. Pelvic pain and fever in women — pelvic inflammatory disease (PID — from ascending chlamydia or gonorrhoea). Scrotal pain and swelling — epididymo-orchitis. HIV seroconversion illness (2–4 weeks after infection): glandular fever-like illness with fever, pharyngitis, lymphadenopathy, macular rash, myalgia.

Chlamydia and gonorrhoea: NAAT (nucleic acid amplification test — PCR) is the gold standard; collected from first-catch urine (men) or self-taken low vaginal swab (women); pharyngeal and rectal swabs for MSM and those at anatomical risk. Gonorrhoea culture and sensitivity is also essential to detect antibiotic resistance patterns. Syphilis serology: treponemal IgG/IgM screen (EIA or CLIA) confirmed by RPR/VDRL titre (quantitative — monitors treatment response); TPPA or TPHA for confirmation. HIV: 4th-generation combined antigen-antibody test (Ag/Ab combo — p24 antigen detected as early as 10–14 days post-infection; before seroconversion); repeat at 45 days if initial negative with recent exposure. HSV: viral PCR from active ulcer swab (gold standard for genital ulcer disease); HSV-1 and HSV-2 type-specific IgG serology for past exposure assessment. HPV: primary HPV DNA testing from cervical samples (integrated into UK cervical screening from age 25); genital warts diagnosed clinically. Hepatitis B: HBsAg (active infection), HBsAb (vaccine-induced or natural immunity), HBcAb (past exposure). Annual STI screening recommended for all sexually active adults under 25; 3-monthly for MSM and PrEP users.

Chlamydia: doxycycline 100 mg twice daily for 7 days (BASHH preferred — superior efficacy for rectal chlamydia over azithromycin) or azithromycin 1 g single dose. Test of cure not routinely needed except in pregnancy and rectal infection. Gonorrhoea: ceftriaxone 500 mg IM single dose (BASHH 2019 guideline — rising antimicrobial resistance makes culture and sensitivity testing mandatory for treatment failure cases; previous dual therapy with azithromycin no longer routinely recommended due to AMR). Test of cure required. Syphilis: primary, secondary, and early latent — benzathine penicillin G 2.4 MU IM single dose; late latent — 3 weekly doses; neurosyphilis — IV penicillin for 10–14 days; Jarisch-Herxheimer reaction (fever within hours of first dose) managed with antipyretics. HIV: antiretroviral therapy (ART) started as soon as possible — preferred first-line regimen: bictegravir/tenofovir alafenamide/emtricitabine (Biktarvy) single-tablet; suppresses viral load to undetectable (U=U — Undetectable = Untransmittable). PrEP: tenofovir-emtricitabine daily. HPV: no cure; Gardasil-9 (nine-valent) vaccine prevents 90%+ of cancers; genital warts treated with podophyllotoxin, imiquimod, cryotherapy. HSV: aciclovir 400 mg TDS or valaciclovir 500 mg daily suppressive therapy. Trichomoniasis: metronidazole 2 g single dose (both partners simultaneously).

Untreated or inadequately treated STIs cause severe and often irreversible complications. Pelvic inflammatory disease (PID) from ascending chlamydia or gonorrhoea causes fallopian tube scarring — resulting in infertility (12% after one PID episode, 35% after three), a 6–10-fold increased risk of ectopic pregnancy, and chronic pelvic pain. Epididymo-orchitis in men causes obstruction and reduced fertility. Neonatal infections: ophthalmia neonatorum (gonococcal eye infection causing blindness), congenital syphilis (stillbirth, organ damage, bone disease), neonatal herpes (CNS damage, 30% mortality without treatment), and mother-to-child HIV transmission (7–40% without antiretroviral prophylaxis). Cervical and oropharyngeal cancers: HPV types 16 and 18 cause 99% of cervical cancers; HPV is also the leading cause of anal, penile, vulvar, and oropharyngeal cancers. HIV immunodeficiency: untreated HIV causes progressive CD4 depletion — AIDS (CD4 below 200 cells/µL) with fatal opportunistic infections (PCP pneumonia, CMV, toxoplasmosis). Neurosyphilis and cardiovascular syphilis (aortitis) in late untreated disease. Hepatitis B leads to cirrhosis and hepatocellular carcinoma in 15–25% of chronically infected individuals.

Correct and consistent male or female condom use reduces transmission of most bacterial STIs (chlamydia, gonorrhoea) by 80–90% and HIV by approximately 70–85% per act. HIV PrEP (tenofovir-emtricitabine — Truvada or Descovy) for high-risk individuals — daily PrEP reduces HIV acquisition by 99%+ with consistent adherence; available free on NHS in England and Scotland; event-based PrEP (2-1-1 method) is effective for MSM with planned sexual encounters. HPV vaccination: Gardasil-9 nine-valent vaccine for ages 9–26, ideally before sexual debut — prevents 90%+ of HPV-related cancers; catch-up vaccination up to age 45 in some guidelines. Hepatitis B vaccination: three-dose highly effective vaccine recommended for unvaccinated adults at risk. Regular STI screening: all sexually active adults under 25 annually; MSM every 3 months; PrEP users every 3 months. HIV PEP (post-exposure prophylaxis): must be started within 72 hours of unprotected sex with HIV-positive partner — reduces HIV acquisition by 80%+. Partner notification and simultaneous treatment to prevent reinfection.

Visit a sexual health clinic or see your GP if you notice: unusual discharge from the penis, vagina, or rectum; sores, ulcers, blisters, or rashes on or around the genitals, mouth, or anus; burning or pain on urination; unusual scrotal or testicular pain or swelling; or if a sexual partner has been diagnosed with an STI. Many STIs (including chlamydia, gonorrhoea, syphilis, and HIV) are asymptomatic — annual screening for all sexually active adults under 25 and 3-monthly screening for MSM is recommended. Seek urgent assessment for: fever with pelvic pain in women (pelvic inflammatory disease — delay in treatment risks fallopian tube damage and infertility); redness, pain, and swelling of the glans penis (balanitis — may be gonococcal); acute painful genital ulcers in a person with fever and lymphadenopathy (primary HSV or chancroid); and any presentation suggesting HIV seroconversion illness — fever, sore throat, rash, lymphadenopathy, and diarrhoea 2–4 weeks after a high-risk sexual exposure (urgent HIV RNA testing recommended).