Common Sexually Transmitted Infections (STIs)

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Common Sexually Transmitted Infections STIs (Most Common → Least Common)

A sexually transmitted infection (sexually transmitted disease) is a serious condition that can develop after you have sex. Common STI symptoms include itching and burning around your genital area. The good news is that most STI treatments can cure the infection, but not all types. You can get an STI again, even after treatment to cure it.

Based on the BASHH guidelines UK sexual Health practice. Exact rankings vary by year and population, but in UK clinics the approximate order is: The order will be different in the USA, Canada, Asia, Africa, Caribbean, South America & the rest of Europe.

  1. Chlamydia
  2. Genital Warts (HPV)
  3. Genital Herpes (HSV)
  4. Gonorrhoea
  5. Mycoplasma genitalium
  6. Trichomonas vaginalis
  7. Syphilis
  8. Molluscum contagiosum

Note: Pelvic Inflammatory Disease (PID) is not an STI itself. It is a complication caused by STIs, especially chlamydia and gonorrhoea.

Sexually Transmitted Infections (STIs): Symptoms, Testing and Treatment

Sexually transmitted infections (STIs) are very common. Many people with an STI have no symptoms at all, which is why testing is so important.

The good news is that most STIs can be treated effectively, and many can be cured completely.

Common Sexually Transmitted Infections STIs

1. Chlamydia – The Most Common Curable Bacterial STI

Key Facts

  • Chlamydia is the most commonly reported curable bacterial STI in the UK.
  • It is most common in people aged 15–24 years.
  • Up to 75% of sexual partners may also be infected.
  • Up to 50% of untreated infections may clear naturally within about 12 months.
  • Most infected women have no symptoms.

Symptoms

Women

  • No symptoms in most cases
  • Vaginal discharge
  • Bleeding after sex
  • Bleeding between periods
  • Pain when passing urine
  • Pelvic pain
  • Pain during sex

Men

  • Discharge from the penis
  • Pain when passing urine

How Is It Diagnosed?

  • NAAT (nucleic acid amplification test)
  • Vaginal swab in women
  • First-catch urine in men

Treatment

  • Doxycycline for 7 days is first-line treatment.
  • Sexual partners should be tested and treated.
  • Avoid sex until treatment is completed.

Why It Matters

Untreated chlamydia can cause:

  • Pelvic inflammatory disease (PID)
  • Infertility
  • Ectopic pregnancy

2. Genital Warts (HPV)

Key Facts

  • Caused by low-risk strains of Human Papillomavirus (HPV).
  • One of the commonest viral STIs worldwide.
  • Different HPV types cause cervical cancer; the types causing warts rarely cause cancer.

Symptoms

  • Small flesh-coloured growths around:
    • Vulva
    • Penis
    • Anus

Diagnosis

  • Usually diagnosed by examination alone.

Treatment

  • Some disappear without treatment.
  • Creams such as imiquimod or podophyllotoxin.
  • Freezing treatment (cryotherapy).
  • Surgical removal if needed.

3. Genital Herpes

Key Facts

  • Caused by Herpes Simplex Virus (HSV).
  • Once infected, the virus remains in the body for life.
  • Symptoms may return (recurrent herpes).

Symptoms

  • Painful blisters
  • Ulcers
  • Burning sensation
  • Pain when passing urine
  • Flu-like symptoms during the first episode

Diagnosis

  • PCR swab from a blister or ulcer.

Treatment

  • Aciclovir or valaciclovir.
  • Pain relief.
  • Suppressive treatment may help frequent recurrences.

4. Gonorrhoea

Key Facts

  • Gonorrhoea rates have increased significantly in recent years.
  • Over 90% of men with urethral infection develop symptoms.
  • About 50% of women with cervical infection develop vaginal discharge.
  • Up to one-third of women with genital gonorrhoea also have rectal infection.

Symptoms

Women

  • Vaginal discharge
  • Pelvic pain
  • Pain when passing urine

Men

  • Discharge from the penis
  • Pain when passing urine

Rectal Infection

  • Usually no symptoms

Throat Infection

  • Usually no symptoms

Diagnosis

  • NAAT test
  • Culture for antibiotic sensitivity

Treatment

  • Ceftriaxone 1 g injection as a single dose.
  • Test of cure recommended.
  • No sex for 7 days after treatment and partner treatment.

Why It Matters

Untreated gonorrhoea can cause:

  • PID
  • Infertility
  • Epididymo-orchitis
  • Disseminated infection affecting joints and skin

5. Mycoplasma genitalium

Key Facts

  • Increasingly recognised as an important STI.
  • Co-infection with chlamydia occurs in approximately 3–15% of cases.
  • Antibiotic resistance is becoming more common worldwide.

Symptoms

Women

  • Cervicitis
  • Pelvic pain
  • Abnormal discharge

Men

  • Urethritis
  • Discharge
  • Pain when passing urine

Diagnosis

  • Specific NAAT test

Treatment

  • Usually doxycycline followed by:
    • Azithromycin if susceptible
    • Moxifloxacin if resistant

Why It Matters

Can contribute to:

  • PID
  • Persistent genital symptoms
  • Ongoing transmission

6. Trichomoniasis (Trichomonas vaginalis)

Key Facts

  • 10–50% of infected women have no symptoms.
  • 15–50% of infected men have no symptoms.
  • Vaginal discharge occurs in up to 70% of infected women.
  • The classic frothy yellow discharge occurs in only 10–30%.
  • “Strawberry cervix” occurs in approximately 2%.

Symptoms

Women

  • Vaginal discharge
  • Vaginal itching
  • Unpleasant smell
  • Pain when passing urine

Men

  • Usually symptom-free
  • Urethral discharge
  • Dysuria

Diagnosis

  • NAAT test (most accurate).
  • Microscopy may also identify the parasite.

Treatment

  • Metronidazole.
  • Sexual partners should also be treated.

7. Syphilis

Key Facts

  • Syphilis cases have risen in many countries.
  • Infection progresses through several stages if untreated.

Symptoms

Primary Syphilis

  • Painless ulcer (chancre)

Secondary Syphilis

  • Rash
  • Mouth ulcers
  • Swollen glands

Late Syphilis

  • Brain disease
  • Heart disease
  • Nerve damage

Diagnosis

  • Blood tests

Treatment

  • Penicillin injections.
  • Follow-up blood tests required.

Why It Matters

Untreated syphilis can cause life-threatening complications years later.

8. Molluscum Contagiosum

Key Facts

  • Caused by a poxvirus.
  • Common in children and sexually active adults.
  • Usually harmless and self-limiting.

Symptoms

  • Small pearly bumps with a central dimple.

Diagnosis

  • Clinical examination.

Treatment

  • Often no treatment required.
  • Cryotherapy or curettage if troublesome.

Pelvic Inflammatory Disease (PID)

What Is PID?

PID is an infection of the female reproductive organs.

It is commonly caused by:

  • Chlamydia
  • Gonorrhoea
  • Mycoplasma genitalium

Symptoms

  • Lower abdominal pain
  • Pelvic tenderness
  • Fever
  • Pain during sex
  • Abnormal discharge

Treatment

Typical treatment includes:

  • Ceftriaxone injection
  • Doxycycline for 14 days
  • Metronidazole for 14 days

Why It Matters

Untreated PID can lead to:

  • Infertility
  • Chronic pelvic pain
  • Ectopic pregnancy

Key Take-Home Messages

✅ Many STIs cause no symptoms.

✅ Testing is the only way to know for sure.

✅ Most STIs are treatable.

✅ Early treatment reduces the risk of infertility and pregnancy complications.

✅ Sexual partners often need testing and treatment too.

✅ Condoms remain one of the best ways to reduce STI transmission.

Remember, ALWAYS consult with your GP and/or your gynecologist for your specific treatment options.

Every woman is different! A visit to an STI Clinic is super important. Your data are anonymised.

Read also: Chancroid, LGV and Granuloma Inguinale: Rare Causes of Genital Ulcers