⚠️ Important Medical Disclaimer
This website provides educational information only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for any health concerns. If you suspect you have an STI, seek immediate medical attention for proper testing and treatment.
Understanding STIs in the UK
More than 30 different bacteria, viruses, and parasites are known to be transmitted through sexual contact. According to the UK Health Security Agency (UKHSA), approximately 402,000 new STI diagnoses were recorded in England in 2025.
The 8 Major STI Pathogens
🦠 Curable (Bacterial/Parasitic)
- Chlamydia - Bacteria
- Gonorrhoea - Bacteria
- Syphilis - Bacteria
- Trichomoniasis - Parasite
🔬 Manageable (Viral)
- HIV - Virus
- HPV - Virus
- Herpes (HSV) - Virus
- Hepatitis B - Virus
Latest UK Statistics (2025)
Data from UKHSA shows the following trends in England:
| STI | 2024 Diagnoses | 2025 Diagnoses | Change |
|---|---|---|---|
| Chlamydia | 168,889 | ~152,000 | ↓ 10% |
| Gonorrhoea | 71,802 | 63,943 | ↓ 11% |
| Infectious Syphilis | 9,535 | 8,262 | ↓ 13.5% |
| Genital Warts | 1,764 | 1,618 | ↓ 8.3% |
Source: UK Health Security Agency, 2025 Report
Detailed Information on Common STIs
| STI | Cause | Curable? | Key Characteristics |
|---|---|---|---|
| Chlamydia | Chlamydia trachomatis (bacteria) | ✓ Yes | Most common bacterial STI in England; often asymptomatic; can cause PID if untreated |
| Gonorrhoea | Neisseria gonorrhoeae (bacteria) | ✓ Yes | Rising antibiotic resistance; affects throat, rectum, genitals |
| Syphilis | Treponema pallidum (bacteria) | ✓ Yes | "Great Imitator" - multiple stages with varied symptoms; congenital transmission risk |
| Trichomoniasis | Trichomonas vaginalis (parasite) | ✓ Yes | Frothy discharge; most common curable STI in women |
| Genital Herpes | HSV-1 or HSV-2 (virus) | ✗ Manageable | Painful blisters/ulcers; recurrent outbreaks; ~9% of UK STI diagnoses |
| HPV | Human Papillomavirus (virus) | ✗ Preventable via vaccine | Genital warts; cervical cancer risk; NHS HPV vaccination programme available |
| HIV | Human Immunodeficiency Virus | ✗ Lifelong treatment | Immune system damage; U=U principle applies (Undetectable = Untransmittable) |
| Hepatitis B | Hepatitis B Virus | ✗ Manageable | Liver infection; part of routine NHS childhood vaccination schedule |
Microscopic Appearance of STI Pathogens
Note: Actual microscopic images require medical laboratory equipment. Below are detailed scientific descriptions for educational purposes. Diagnostic microscopy is performed in accredited UK laboratories.
🔬 Chlamydia trachomatis
Gram-negative Intracellular Size: 0.3-1.0 μmAppears as small intracellular inclusion bodies within epithelial cells. Does not stain well with Gram stain due to unique cell wall structure. Visible under electron microscopy as elementary bodies (infectious form) and reticulate bodies (replicative form). Elementary bodies are dense, metabolically inactive; reticulate bodies are larger and actively dividing.
🔬 Neisseria gonorrhoeae (Gonorrhoea)
Gram-negative Diplococci Size: 0.6-1.0 μmClassic "kidney bean" shaped gram-negative diplococci appearing in pairs with flattened adjacent sides. Intracellular location within neutrophils is characteristic. Coffee-bean appearance under light microscopy at 1000× magnification with Gram staining shows pink/red paired cocci. Used as diagnostic criterion in UK BASHH guidelines.
🔬 Treponema pallidum (Syphilis)
Spirochete Too thin for Gram stain Size: 6-15 μm lengthSpiral-shaped bacterium too thin to be seen with standard light microscopy (0.1-0.2 μm wide). Requires dark-field microscopy showing characteristic corkscrew motility. Multiple tight spirals with regular spacing. Cannot be cultured on artificial media. Silver stain (Warthin-Starry) shows black spirochetes against light background. Rarely used now in favour of serological testing.
🔬 Trichomonas vaginalis
Protozoan parasite Motile Size: 10-20 μmPear-shaped flagellated protozoan visible under wet mount microscopy. Has 4 anterior flagella and one posterior flagellum. Undulating membrane visible along lateral edge. Active jerky motility characteristic. Nucleus at anterior end. Approximately 2-3× size of white blood cells. Sensitivity of wet mount ~60%; NAAT preferred in UK practice.
🔬 Herpes Simplex Virus (HSV)
Enveloped DNA virus Size: 150-200 nmVirus particles (virions) visible only under electron microscopy showing enveloped icosahedral capsid with glycoprotein spikes. Tzanck smear may show multinucleated giant cells and eosinophilic intranuclear inclusions (Cowdry type A). Viral cytopathic effect causes cell rounding and fusion. PCR testing preferred in UK clinical practice.
🔬 Human Papillomavirus (HPV)
Non-enveloped DNA virus Size: 52-55 nmSmall non-enveloped icosahedral virus visible only via electron microscopy. Cervical biopsy may show koilocytes - squamous epithelial cells with enlarged hyperchromatic nuclei surrounded by large perinuclear halos (pathognomonic for HPV). No characteristic blood smear findings. Cervical screening (smear test) detects cellular changes.
What Men and Women Notice When Infected
Important: Most STIs are asymptomatic (show no symptoms). Regular screening is essential for sexually active individuals. According to UK data, up to 70% of chlamydia cases in women and 50% in men are asymptomatic.
Chlamydia
👨 In Men
- Clear or milky discharge from penis tip
- Burning sensation during urination
- Pain/swelling in testicles (rare)
- Itching or irritation at urethra opening
- ↑ Up to 50% of cases are asymptomatic
👩 In Women
- Abnormal vaginal discharge
- Burning during urination
- Abdominal/pelvic pain (if PID develops)
- Pain during intercourse
- Bleeding between periods or after sex
- ↑ Up to 70-80% of cases are asymptomatic
Gonorrhoea
👨 In Men
- Thick white/yellow/green penile discharge
- Burning during urination (dysuria)
- Increased urination frequency
- Painful or swollen testicles (less common)
- Redness at urethral opening
👩 In Women
- Increased vaginal discharge
- Urinary burning or urgency
- Lower abdominal pain (sign of PID)
- Fever (if PID present)
- ↑ 80%+ are asymptomatic initially
Syphilis (Multiple Stages)
👨 In Men
- Primary: Single painless ulcer (chancre) on penis - appears 3 weeks after exposure
- Secondary: Rash on palms/soles, fever, swollen lymph nodes, sore throat, patchy hair loss
- Latent: No symptoms (can last years)
- Late: Neurological/cardiovascular problems (years later if untreated)
👩 In Women
- Primary: Painless ulcer on cervix or vulva (often unnoticed internally)
- Secondary: Same rash (palms/soles), hair loss, fatigue, flu-like symptoms
- Warning: Can transmit to fetus causing congenital syphilis
- Pregnancy: All pregnant women tested at booking appointment
Genital Herpes (HSV-1/HSV-2)
👨 In Men
- Clusters of painful blisters on penis or scrotum
- Ulcers that crust over when blisters burst
- Prodrome: tingling/burning/shooting pains before outbreak
- Recurrent episodes during stress/illness
- Fever/muscle aches during first outbreak
👩 In Women
- Painful blisters on labia, vagina, or cervix
- Unusual vaginal discharge
- Difficult/painful urination (when urine touches sores)
- Outbreaks on buttocks or thighs possible
- Cervical involvement (internal)
Human Papillomavirus (HPV)
Most HPV infections are asymptomatic and clear spontaneously within 2 years
👨 In Men
- Small bumps or clusters on penis, scrotum, or anus
- Flattened warts (may resemble cauliflower)
- Warts on thigh or groin area
- Most cases show no visible symptoms
- Oropharyngeal cancer risk (certain strains)
👩 In Women
- Warts on vulva, vagina, cervix, or anus
- Usually discovered via cervical screening (smear test)
- Irregular bleeding (if cervical lesions present)
- Cervical cancer risk with high-risk strains (16, 18)
- Most infections remain subclinical
Treatment Options by STI (UK Guidelines)
All treatment recommendations are based on current BASHH (British Association for Sexual Health and HIV) and NHS guidelines (2024-2025).
Curable Bacterial/Parasitic STIs
🦠 Chlamydia Treatment
First-line: Doxycycline 100 mg orally twice daily for 7 days
Alternative (pregnancy/allergy): Azithromycin 1 g single dose
Pregnancy: Azithromycin 1 g single dose preferred
Abstain from sex: 7 days after treatment completion
Partner notification: All partners within 6 months must be tested and treated
Follow-up: Retest in 3 months recommended (reinfection rates high)
NHS service: Free at GP, sexual health clinics, or via self-test kit
🦠 Gonorrhoea Treatment
First-line (BASHH 2025): Ceftriaxone 1 g IM single dose
Weight ≥150 kg: Ceftriaxone 2 g IM single dose
Co-treatment: Also treat for chlamydia unless ruled out (unless doxycycline contraindicated)
Antibiotic resistance: Extensively documented in UK; azithromycin resistance rising
Test of cure: Mandatory for all sites within 2 weeks post-treatment
Partner notification: All partners within 2 months must be notified
🦠 Syphilis Treatment
Primary/Secondary/Early Latent (<1 year):
• Benzathine Penicillin G 2.4 million units IM single dose
Late Latent (>1 year) or Unknown duration:
• Benzathine Penicillin G 2.4 million units IM weekly × 3 doses
Penicillin-allergic (non-pregnant):
• Doxycycline 100 mg twice daily × 14 days (early) or 28 days (late)
DoxyPEP: New UK guideline (2025) for post-exposure prophylaxis in high-risk groups
Jarisch-Herxheimer reaction: Expected fever/chills 2-24 hrs post-treatment - common in primary/secondary
🦠 Trichomoniasis Treatment
Women: Metronidazole 400 mg twice daily × 7 days (preferred) OR 2g single dose
Men: Metronidazole 2g single dose
Partner treatment: Essential - all recent partners must be treated simultaneously
Alcohol: Avoid alcohol during and 48 hours after metronidazole
Retesting: Test again in 3 months due to high reinfection rates
Viral STIs (Manageable, Not Curable)
🔬 Genital Herpes (HSV) Management
Episodic treatment (first episode):
- Aciclovir 400 mg three times daily × 5-10 days
- Valaciclovir 500 mg twice daily × 5 days
- If very severe: consider IV aciclovir
Recurrent outbreaks:
- Aciclovir 400 mg three times daily × 5 days
- Valaciclovir 500 mg twice daily × 3 days
Suppressive therapy (≥6 episodes/year):
- Valaciclovir 500mg once daily
- Aciclovir 400mg twice daily
Pregnancy: Daily antiviral prophylaxis from 36 weeks gestation; C-section if active lesions at delivery
🔬 HPV Management
No cure for viral infection itself
NHS Vaccination Programme: Gardasil 9 offered routinely to:
- Girls and boys aged 12-13 years (School Year 8)
- MSM up to age 45 via GUM/HIV clinics
- Immunocompromised individuals up to age 45
External warts treatment options:
- Podophyllotoxin cream (patient-applied)
- Imiquimod cream (immune response modifier)
- Cryotherapy (freezing) at clinic
- Trichloroacetic acid (clinic application)
Cervical screening: HPV primary testing now standard in NHS Cervical Screening Programme
🔬 HIV Treatment
Requires lifelong antiretroviral therapy (ART)
Standard regimen: Integrase strand transfer inhibitor (INSTI)-based triple therapy
- Biktarvy (Bictegravir + Tenofovir DF + Emtricitabine)
- Dovato (Dolutegravir + Lamivudine) - dual therapy option
U=U Principle: Undetectable = Untransmittable - with proper ART, HIV cannot be sexually transmitted (BHIVA guideline)
PrEP available on NHS: Free for eligible individuals via sexual health clinics
PEPSE (Post Exposure Prophylaxis): Available within 72 hours after high-risk exposure (A&E or sexual health clinic)
Regular monitoring: Viral load + CD4 count checks every 3-6 months
🔬 Hepatitis B Management
Acute infection: Usually supportive care (most clear spontaneously)
Chronic infection treatment:
- Tenofovir disoproxil fumarate (TDF)
- Tenofovir alafenamide (TAF)
- Entecavir
Monitoring: Liver function tests, viral load, liver ultrasound
Prevention: Part of routine NHS childhood immunisation schedule since 2024 (combined with other vaccines)
Prevention Strategies (UK Guidance)
- Consistent condom use reduces STI transmission risk by 80-95%
- NHS HPV vaccination - offered routinely to 12-13 year olds; catch-up available
- NHS Hepatitis B vaccination - routine since 2024; also available for at-risk adults
- PrEP on the NHS - free HIV prevention for eligible individuals via sexual health clinics
- Regular screening for sexually active individuals (annual minimum)
- Mutual monogamy with tested partner reduces risk significantly
- Partner notification - crucial to prevent reinfection; clinics offer anonymous contact tracing
- PEPSE (Post Exposure Prophylaxis) available within 72 hours for HIV exposure (contact A&E or sexual health clinic urgently)
- DoxyPEP (2025) - doxycycline post-exposure prophylaxis now recommended for high-risk groups (BASHH guideline)
Recommended Screening Schedule (NHS/BASHH)
| Population | Recommended Tests | Frequency |
|---|---|---|
| Sexually active under 25s | Chlamydia, Gonorrhoea (National Chlamydia Screening Programme) | Annually or on change of partner |
| MSM (gay, bisexual, other men who have sex with men) | HIV, Syphilis, Gonorrhoea, Chlamydia (throat, rectum, urine) | Every 3 months (every 6 months if stable partner) |
| Multiple partners / casual partners | Full STI panel including HIV, Hepatitis B/C | Every 3-6 months |
| Pregnant women | HIV, Syphilis, Hepatitis B, Chlamydia, Gonorrhoea | First antenatal booking appointment + repeat as indicated |
| Age 25-49 (women) | Cervical screening (HPV primary testing) | Every 3-5 years depending on age/results |
| HIV-positive individuals | Full STI screen | Every 3-6 months |
Where to Get Tested & Treatment in the UK
All NHS sexual health services are confidential and free regardless of nationality, immigration status, or ability to pay.
🏥 Healthcare Options
- GP Surgery - Some provide sexual health tests
- Specialist Sexual Health Clinics (GUM clinics)
- Contraceptive Clinics
- University/College Health Services
- Some Pharmacies - chlamydia self-testing/treatment
- A&E - for emergencies or PEPSE access
📱 NHS Testing Services
- Online ordering - self-test kits posted to home
- NHS 111 - advice and local service finder
- Find a clinic tool - NHS.uk service search
- Brook - free confidential advice for under 25s
- Terrence Higgins Trust - HIV/sexual health support
💊 Medication & Regulation
- All medications approved by MHRA
- Treatment free via NHS prescription or clinic
- Private prescriptions available
- Report adverse reactions via Yellow Card Scheme
🚨 When to Seek Urgent Medical Attention
Contact your GP, sexual health clinic, or call NHS 111 urgently if you experience:
- Severe pelvic/abdominal pain (possible Pelvic Inflammatory Disease - PID)
- Fever with genital symptoms
- Yellowing of skin/eyes (jaundice) - possible Hepatitis
- Confusion, severe headache, vision changes (possible neurosyphilis)
- Difficulty breathing or swallowing after suspected exposure
- Severe allergic reaction to treatment medications
- New HIV exposure within last 72 hours (seek PEPSE immediately)
For PEPSE outside clinic hours: Go to nearest A&E or call NHS 111
Key UK Organisations & Resources
🇬🇧 Official Bodies
- UKHSA - UK Health Security Agency (surveillance, data)
- NHS - National Health Service (testing, treatment)
- BASHH - British Association for Sexual Health and HIV (guidelines)
- MHRA - Medicines & Healthcare products Regulatory Agency
🤝 Charities & Support
- Terrence Higgins Trust - HIV, sexual health, hepatitis
- Brook - Under 25s sexual health
- FPA (Family Planning Association)
- Stonewall - LGBTQ+ health resources
- GMFA - Men's sexual health