Sexually transmitted infections: overview

Scale

WHO estimates more than 1 million curable STIs are acquired every day worldwide, with about 374 million new infections a year of chlamydia, gonorrhoea, syphilis and trichomoniasis. Most are asymptomatic. Roughly 70 percent of chlamydia infections in women and 50 percent in men produce no symptoms, which is why symptom-based screening misses most cases and why routine testing is the only reliable detection method.

Bacterial infections — curable

Chlamydia. The most commonly diagnosed bacterial STI. Usually silent. Untreated, it can cause pelvic inflammatory disease, tubal infertility, ectopic pregnancy and epididymitis. Treated with doxycycline 100 mg twice daily for 7 days, now preferred over single-dose azithromycin because of better cure rates, especially rectal.

Gonorrhoea. Can infect urethra, cervix, rectum, throat and eyes. Throat infections are almost always asymptomatic and are a major reservoir; they are also the hardest to cure. Antimicrobial resistance is the dominant concern — WHO classifies N. gonorrhoeae as a high-priority pathogen, resistance to ceftriaxone has been reported in multiple countries, and first-line treatment is now higher-dose ceftriaxone with test-of-cure for pharyngeal infection.

Syphilis. Progresses in stages: a painless chancre (often unnoticed, and often internal), then a rash frequently involving palms and soles, then a latent phase, then potentially neurological, cardiac and ocular disease years later. Congenital syphilis causes stillbirth and severe infant disease and has risen sharply in the US and parts of Europe since 2015. Curable with benzathine penicillin at every stage; earlier treatment prevents more damage.

Mycoplasma genitalium. Increasingly recognised, causes urethritis and cervicitis, and has high macrolide resistance. Testing is not universally available and BASHH advises resistance-guided therapy where it is.

Viral infections — manageable, mostly not curable

HIV. Treatable and, on effective treatment, not sexually transmissible. The PARTNER and PARTNER2 studies followed serodifferent couples through roughly 128,000 acts of condomless sex where the partner with HIV had a viral load under 200 copies/mL and recorded zero phylogenetically linked transmissions. This is the evidence base for Undetectable = Untransmittable. Life expectancy on modern treatment approaches that of the general population.

Herpes simplex (HSV-1 and HSV-2). Extremely common: WHO estimates around 3.8 billion people under 50 have HSV-1 and around 520 million aged 15–49 have HSV-2. Most are unaware. Genital herpes is increasingly caused by HSV-1 via oral sex. It sheds asymptomatically, which is how most transmission occurs. Antivirals shorten outbreaks, and daily suppressive valaciclovir reduces transmission to a partner by roughly 48 percent (Corey et al., 2004, NEJM).

HPV. So common it is close to universal in sexually active people; most infections clear within two years. Persistent high-risk types cause cervical, anal, penile, vulval, vaginal and oropharyngeal cancers. Vaccination is highly effective — Lei et al. (2020, NEJM), following 1.7 million Swedish women, found substantially reduced invasive cervical cancer among those vaccinated, with the largest reduction in those vaccinated before 17.

Hepatitis B. Vaccine-preventable and sexually transmissible; more infectious than HIV by the same routes. Hepatitis C is transmitted mainly by blood but sexual transmission occurs, particularly with condomless anal sex among men with HIV and in chemsex settings. Hepatitis C is now curable in most cases with 8–12 weeks of direct-acting antivirals.

Mpox. Transmitted by close skin-to-skin contact including sexual contact; vaccination is offered to higher-risk groups in many countries.

Parasitic

Trichomoniasis — curable with metronidazole, frequently asymptomatic in men. Pubic lice and scabies — treatable topically; bedding and clothing need washing.

Facts that get missed

• STIs pass through oral sex. Gonorrhoea and syphilis of the throat are common; HSV-1 causes genital herpes.
• STIs pass between people of any gender. Woman-to-woman transmission of HPV, herpes, trichomoniasis and syphilis is documented, and lesbian and bisexual women are under-screened.
• Being on PrEP prevents HIV only, and correlates in several cohorts with higher rates of other STIs where condom use drops.
• Having one STI increases the chance of acquiring HIV, through mucosal inflammation.
• There is no "clean" and "dirty". Having an STI is a medical event, not a character assessment.

Sources

• WHO. "Sexually transmitted infections (STIs)" fact sheet and global surveillance reports, current edition.
• Rodger AJ et al. "Risk of HIV transmission through condomless sex in serodifferent gay couples with the HIV-positive partner taking suppressive antiretroviral therapy (PARTNER2)." Lancet, 2019.
• Corey L et al. "Once-daily valacyclovir to reduce the risk of transmission of genital herpes." NEJM, 2004.
• Lei J et al. "HPV vaccination and the risk of invasive cervical cancer." NEJM, 2020.
• CDC. Sexually Transmitted Infections Treatment Guidelines, 2021 and updates.
• BASHH. UK national guidelines for individual infections, current editions.
• WHO. "Global priority list of antibiotic-resistant bacteria," and gonorrhoea AMR surveillance (EGASP).

Last reviewed: August 2026.