Sexually Transmitted Infections (STIs) — Causes, Symptoms & Treatment Guide — Symptoms, Causes & Treatment | MyMedicPlus

Sexually transmitted infections (STIs) — also called sexually transmitted diseases (STDs) or venereal diseases — are infections transmitted primarily through sexual contact, including vaginal, anal, and oral sex. The WHO estimates more than 1 million new curable STIs are acquired daily worldwide, with 374 million new cases of the four most common curable STIs (chlamydia, gonorrhoea, syphilis, and trichomoniasis) occurring annually. STIs encompass over 30 different bacteria, viruses, and parasites. The most common include: bacterial — chlamydia (Chlamydia trachomatis), gonorrhoea (Neisseria gonorrhoeae), syphilis (Treponema pallidum); viral — HIV (human immunodeficiency virus), HPV (human papillomavirus — most common STI globally), herpes simplex virus (HSV-1, HSV-2), hepatitis B virus (HBV); parasitic — Trichomonas vaginalis, pubic lice (Phthirus pubis), scabies. Many STIs are curable with antibiotics when detected early; viral STIs are generally manageable but not curable (except hepatitis C). Untreated STIs cause significant complications including infertility, ectopic pregnancy, neonatal disease, cervical cancer (HPV), and HIV transmission.

STIs are caused by pathogens transmitted during sexual activity: bacteria (chlamydia, gonorrhoea, syphilis, chancroid), viruses (HIV, HPV, HSV, HBV, HCV — though HCV primarily blood-borne), and parasites (Trichomonas). Transmission routes: vaginal sex (most STIs), anal sex (highest HIV transmission risk — 18 times higher receptive risk vs. vaginal sex per act), oral sex (herpes, gonorrhoea, syphilis, HPV), skin-to-skin contact (HSV, HPV, syphilis, molluscum), blood-to-blood (HIV, HBV, HCV — sharing needles, blood transfusion), mother-to-child (vertical transmission — HIV, syphilis, herpes, gonorrhoea — during pregnancy, childbirth, or breastfeeding). Risk factors: multiple sexual partners (each new partner brings cumulative exposure), inconsistent condom use, young age (under 25 — highest chlamydia prevalence), men who have sex with men (MSM — higher risk of HIV, syphilis, gonorrhoea, HPV), sex workers and their clients, IV drug use (blood-borne STIs), alcohol and substance use (impairs risk judgement), history of prior STI (biological susceptibility and behavioural factors), lack of STI screening and testing, and low access to healthcare. Global antimicrobial resistance is an emerging crisis — gonorrhoea (Neisseria gonorrhoeae) has developed resistance to penicillin, fluoroquinolones, and is developing resistance to cephalosporins — the WHO classified multi-drug resistant gonorrhoea as a priority pathogen.

Many STIs are asymptomatic — most chlamydia infections (70% in women, 50% in men), many gonorrhoea infections, and early HIV and syphilis can be silent, which is why screening is essential. When symptoms occur, they vary by pathogen. Urethral and cervical symptoms (chlamydia, gonorrhoea): urethral discharge (yellow-green, purulent in gonorrhoea; clear-white in chlamydia), dysuria (pain on urination), intermenstrual or post-coital bleeding in women, pelvic pain. Genital ulcers: painful ulcers — herpes simplex (multiple small vesicles progressing to painful ulcers; primary episode with fever and lymphadenopathy); chancroid (Haemophilus ducreyi — soft, painful ulcer with inguinal lymphadenopathy). Painless ulcer: syphilitic chancre (primary syphilis — solitary, indurated, painless ulcer at the site of inoculation; usually resolves without treatment but disease progresses). Secondary syphilis: generalised rash (including palms and soles), condylomata lata (warty lesions), flu-like symptoms, lymphadenopathy. Warts: genital warts (HPV 6, 11) — cauliflower-like papules on genitalia and perianal area. Vaginal symptoms (trichomoniasis, BV, Candida): frothy yellow-green offensive discharge (Trichomonas); grey homogeneous discharge with fishy odour (bacterial vaginosis — Gardnerella — though BV is not strictly an STI); thick white cottage-cheese discharge (candidiasis). HIV seroconversion illness (2–4 weeks post-infection): glandular fever-like illness — fever, pharyngitis, rash, lymphadenopathy, myalgia — resolves spontaneously; disease then enters latent phase lasting years.

A comprehensive STI screen typically includes: nucleic acid amplification test (NAAT — PCR) for chlamydia and gonorrhoea (from urine, self-collected swabs, or clinician-taken urogenital, rectal, and pharyngeal swabs); syphilis serology (treponemal IgG/IgM screen — CLIA or EIA with RPR titre for staging and treatment response — RPR falls with effective treatment); HIV 4th-generation antigen-antibody combination test (Ag/Ab combo — detects p24 antigen early in infection, before antibody seroconversion; if negative but recent exposure suspected, repeat at 45 days); hepatitis B surface antigen (HBsAg) and hepatitis B surface antibody (HBsAb — for vaccination status); hepatitis C antibody (anti-HCV) with reflex PCR. Herpes simplex: viral PCR from ulcer swab (gold standard for active lesion); serology (HSV-1 and HSV-2 IgG antibodies — indicates past infection). HPV: cervical HPV DNA testing integrated into cervical cancer screening programmes (primary HPV testing in the UK from age 25); genital warts diagnosed clinically. Microscopy: Trichomonas vaginalis — motile trichomonads on wet preparation (50–70% sensitivity) or NAAT (preferred). Sexual contact tracing (partner notification): all confirmed STI cases must undergo partner notification — contacts tested and treated if positive.

Chlamydia: doxycycline 100 mg twice daily for 7 days (preferred — superior efficacy in rectal chlamydia) or azithromycin 1 g single dose (but higher rectal chlamydia failure rates). Test of cure not routinely required. Partners tested and treated. Gonorrhoea: ceftriaxone 500 mg IM single dose (UK BASHH guideline 2019) — due to increasing antibiotic resistance, dual therapy with azithromycin 1 g was previously used but ceftriaxone monotherapy now recommended for urogenital gonorrhoea; culture and sensitivity mandatory for treatment failure. Test of cure required. Syphilis: primary, secondary, and early latent syphilis — benzathine penicillin G 2.4 MU IM single dose; late latent or unknown duration — 3 weekly doses. Neurosyphilis: IV crystalline penicillin for 10–14 days. Doxycycline or azithromycin for penicillin allergy (lower evidence). Jarisch-Herxheimer reaction (fever, sweating within hours of treatment) — treat with antipyretics; warn patients in advance. HIV: antiretroviral therapy (ART) — initiated as soon as possible after diagnosis regardless of CD4 count. Preferred first-line: tenofovir alafenamide (TAF) + emtricitabine (FTC) + bictegravir (integrase inhibitor) single-tablet regimen — suppresses viral load to undetectable, preventing transmission (U=U — Undetectable = Untransmittable). HIV pre-exposure prophylaxis (PrEP): tenofovir-emtricitabine (Truvada or Descovy) daily or event-based (2-1-1 method for MSM) — reduces HIV acquisition by 99%+ with daily adherence. Herpes simplex: aciclovir 400 mg TDS for 5–7 days for primary episode; episodic treatment or daily suppressive therapy (valaciclovir 500 mg OD) for recurrent disease. HPV warts: topical podophyllotoxin 0.5% (patient-applied), imiquimod 5% cream, cryotherapy, or electrocautery. Trichomoniasis: metronidazole 2 g single dose (or 400 mg BD for 7 days — both partners treated simultaneously).

Untreated or inadequately treated STIs cause a wide spectrum of serious complications. Pelvic inflammatory disease (PID): ascending infection from chlamydia or gonorrhoea into the fallopian tubes, uterus, and ovaries — causes chronic pelvic pain, tubal scarring leading to infertility (in 12% after one episode, 35% after three episodes), and a 6-10 fold increased risk of ectopic pregnancy. Infertility: both men and women — gonorrhoea causes epididymo-orchitis in men (risk of obstructive azoospermia); PID causes tubal factor infertility in women. Neonatal complications: ophthalmia neonatorum (gonococcal eye infection — blindness if untreated), congenital syphilis (bone and organ damage, stillbirth), neonatal herpes (CNS involvement — 30-40% mortality without treatment), and mother-to-child HIV transmission (7-40% without antiretrovirals). Cervical and oropharyngeal cancer: high-risk HPV types 16 and 18 cause 99% of cervical cancers, 90% of anal cancers, 70% of oropharyngeal cancers, and significant proportions of vulvar, vaginal, and penile cancers. HIV AIDS: untreated HIV leads to progressive CD4 lymphocyte depletion — AIDS develops when CD4 count falls below 200 cells/microlitre, causing AIDS-defining illnesses (Pneumocystis jirovecii pneumonia, CMV retinitis, CNS toxoplasmosis). Syphilis complications: neurosyphilis (meningitis, dementia, tabes dorsalis), cardiovascular syphilis (aortitis, aortic aneurysm). Hepatitis B-related cirrhosis and hepatocellular carcinoma.

Condom use correctly and consistently reduces transmission of bacterial STIs by 80–90% and HIV by approximately 70–85% per act. Male condoms and female (internal) condoms are effective barriers. Regular STI screening: all sexually active people under 25 should be tested annually for chlamydia (UK NHS chlamydia screening programme); MSM should be screened every 3 months for HIV, syphilis, gonorrhoea, and chlamydia; HIV-positive individuals — annual screening for other STIs. HIV PrEP: daily tenofovir-emtricitabine reduces HIV risk by 99%+ in high-risk individuals — available free on NHS in England and Scotland; available in most high-income countries. HPV vaccination: Gardasil 9 (nine-valent HPV vaccine) protects against HPV types 6, 11, 16, 18, 31, 33, 45, 52, 58 — preventing 90%+ of cervical cancers and genital warts; recommended for all children aged 9–14 (2-dose schedule); catch-up vaccination to age 26 for unvaccinated individuals. Hepatitis B vaccination: highly effective 3-dose vaccine — recommended universally in infancy and for unvaccinated adults at risk. Partner reduction and mutual monogamy reduce STI risk. Post-exposure prophylaxis (HIV PEP): tenofovir + emtricitabine + raltegravir (or dolutegravir) for 28 days after unprotected sex with HIV-positive partner — must be started within 72 hours; available from GUM clinics and emergency departments.

Seek emergency care or go to a GUM clinic immediately for: a suspected HIV exposure (PEP must be started within 72 hours to be effective — every hour matters), eye discharge in a newborn within the first week of life (gonococcal ophthalmia neonatorum — emergency), or signs of pelvic inflammatory disease (severe pelvic pain, fever, vomiting). See a sexual health clinic or your GP within 1–3 days for: any new genital ulcer or sore, penile or vaginal discharge, unusual rash (especially involving palms and soles — syphilis), genital warts, painful urination, or testicular or scrotal pain (epididymo-orchitis from gonorrhoea or chlamydia). Have an STI screen (including HIV test) if: you have had unprotected sex with a new partner, your partner has been diagnosed with an STI, or you have any symptoms — STI clinics are confidential, non-judgmental, and free in the NHS. Sexual contact tracing is an important public health duty — your clinic will support this confidentially.