Sexually Transmitted Infections

TL;DR. Sexually transmitted infections are ordinary infections with an unusual social load. Biologically they are unremarkable: bacteria, viruses, and parasites that happen to spread through mucosal contact. Most are curable, all are testable, and several are vaccine-preventable. What makes them different is that shame delays testing, delayed testing spreads them further, and the ones that cause the most long-term harm (chlamydia, HPV, syphilis in pregnancy) usually cause no symptoms at all. Over a million curable STIs are acquired every day worldwide, and syphilis, a disease that was nearly eliminated in several countries, has come back hard, taking congenital syphilis with it.

Key takeaways

• WHO estimates more than 1 million curable STIs are acquired each day, mostly asymptomatic.
• Chlamydia is usually silent and is a leading preventable cause of infertility, through scarring of the fallopian tubes.
• Syphilis is resurging. Congenital syphilis, which causes stillbirth, neonatal death, and permanent disability, has risen sharply in several high-income countries, and it is prevented by one blood test and one injection in pregnancy.
• Gonorrhoea has developed resistance to every antibiotic class used against it, in sequence, and is on the WHO priority pathogen list.
• HPV causes cancer and has a vaccine; herpes is common, lifelong, and manageable; hepatitis B is sexually transmitted and vaccine-preventable.
• Testing is the entire strategy, because most transmission comes from people who feel perfectly well.

What they are

In short: Nine ordinary infections with an unusual social load, most curable, all testable, and several causing their worst harm silently.

The history

In short: Syphilis produced the first designed antimicrobial drug and the most cited case in research ethics.

Syphilis appeared explosively in Europe from 1495, first recorded during the French army's siege of Naples, and spread across the continent within years. Every nation named it after a neighbour: the French disease, the Neapolitan disease, the Spanish disease, the Polish disease. Its origin is still debated between a New World introduction after 1492 and a European precursor organism, with skeletal evidence argued on both sides.

Its history shaped medicine more than the disease itself does now:

• Mercury was the standard treatment for four centuries, hence the saying "a night with Venus, a lifetime with Mercury." It was toxic and largely useless.
• Salvarsan (1909), Paul Ehrlich's arsenic compound, was the first designed antimicrobial drug and the origin of chemotherapy as a concept: the search for a "magic bullet" that binds the pathogen and spares the host.
• The Wassermann test (1906) gave the first serological diagnosis.
• Penicillin (1943) cured it definitively and still does. T. pallidum has never developed penicillin resistance, which is close to unique among bacteria.
• The US Public Health Service study at Tuskegee (1932 to 1972) withheld treatment from hundreds of Black men with syphilis, without their informed consent, for decades after penicillin became standard. It is the most cited case in research ethics, produced the Belmont Report and modern institutional review boards, and remains a documented contributor to medical distrust in Black communities in the United States.

Gonorrhoea's history is a resistance timeline: sulfonamides failed in the 1940s, penicillin through the 1970s and 1980s, tetracyclines, then fluoroquinolones in the 2000s, then oral cephalosporins. Ceftriaxone is now the last reliably effective class, and treatment failures have been reported.

What actually goes wrong

In short: Chlamydia scars the fallopian tubes without symptoms, syphilis stages itself over decades, and herpes hides in nerve roots for life.

Mucosal entry. The lining of the genital tract, rectum, and throat is a single layer of cells in places, and it is designed for exchange rather than defence. Micro-abrasions during sex provide entry. This is why receptive anal sex carries the highest transmission risk for most STIs, and why any STI causing ulceration substantially increases HIV transmission in both directions.

Chlamydia and gonorrhoea infect the columnar epithelium of the cervix, urethra, rectum, and throat. The damage is done by the immune response: inflammation ascends to the fallopian tubes, causing pelvic inflammatory disease. Scarring blocks or distorts the tubes, which produces infertility and raises the risk of ectopic pregnancy, a life-threatening implantation outside the uterus. Because up to 70 percent of chlamydia in women is asymptomatic, this damage often accrues silently.

Syphilis is the great imitator, and its staging matters:

Congenital syphilis occurs when the bacterium crosses the placenta. It causes stillbirth, neonatal death, prematurity, bone deformity, deafness, and neurological damage. It is entirely preventable by testing every pregnant woman and giving penicillin, and its resurgence is therefore a system failure rather than a biological one.

HPV infects the basal cells of the epithelium. Most infections clear within one to two years. Persistent infection with high-risk types drives progressive cellular change to precancer and eventually cancer over 10 to 20 years, which is exactly the window screening exploits.

Herpes simplex establishes lifelong latency in the sensory nerve ganglia. It reactivates periodically, travelling back down the nerve to produce lesions, often triggered by stress, illness, or immunosuppression. Crucially, it sheds asymptomatically, so transmission occurs between outbreaks and from people who do not know they carry it. HSV-1 traditionally caused oral herpes and HSV-2 genital, but HSV-1 now causes a large and rising share of genital herpes through oral sex.

What it does to the body

Acute: discharge, pain on urination, ulcers, warts, pelvic pain, testicular pain, sore throat, rectal pain and discharge. Or, very often, nothing at all.

Long-term:

• Infertility in women (tubal factor, from chlamydia and gonorrhoea) and in men (epididymitis leading to obstruction).
• Chronic pelvic pain after pelvic inflammatory disease.
• Ectopic pregnancy, a leading cause of first-trimester maternal death.
• Cancer, from HPV and hepatitis B.
• Neurological and cardiovascular destruction in late syphilis.
• Neonatal disease: congenital syphilis, neonatal herpes (rare and often fatal or disabling), neonatal conjunctivitis from gonorrhoea or chlamydia (historically a major cause of childhood blindness, which is why eye ointment at birth became routine).
• Increased HIV transmission: ulcerative and inflammatory STIs raise the risk of both acquiring and transmitting HIV several-fold.

Is it deadly?

Directly, rarely and specifically: untreated tertiary syphilis, congenital syphilis, ectopic pregnancy, disseminated gonococcal infection, neonatal herpes, and HIV. Indirectly, through cancers caused by HPV and hepatitis B, the toll is large: cervical cancer alone kills over 300,000 women a year.

The bigger burden is morbidity: infertility, chronic pain, pregnancy loss, and the psychological weight of a lifelong diagnosis.

Is it contagious?

Yes, by definition, and the specifics matter.

Transmission occurs through vaginal, anal, and oral sex, through skin-to-skin genital contact (HPV, herpes, syphilis, which is why condoms reduce but do not eliminate their transmission), from mother to child, and, for HIV and hepatitis B, through blood.

Condoms substantially reduce transmission of chlamydia, gonorrhoea, HIV, and trichomoniasis, and partially reduce herpes, HPV, and syphilis, because those can be transmitted from areas the condom does not cover.

They are not transmitted by toilet seats, swimming pools, towels, cutlery, or hugging. This matters because that belief keeps some people from being tested at all.

Who gets it

Age. Rates are highest in people aged roughly 15 to 24, reflecting partner change rates and biological susceptibility (the cervix in adolescence has a more exposed transition zone).

Populations disproportionately affected, and the reasons are structural rather than behavioural in most analyses: men who have sex with men (higher for syphilis, gonorrhoea, and HIV), sex workers, people in prisons, transgender people, and, in the United States, Black and Indigenous populations, where the pattern tracks access to healthcare, sexual network density, and historical distrust rather than any difference in individual behaviour.

Pregnancy is a critical screening moment: syphilis, HIV, hepatitis B, and chlamydia testing in pregnancy prevents the most severe outcomes in this chapter.

Geographically, the highest burdens of curable STIs are in the WHO African Region and in South and Southeast Asia, though surveillance quality varies enough that comparisons should be treated cautiously.

Treatment, and how it works

In short: Most are cured by a short course, partner treatment is part of the treatment, and prevention now includes vaccination and post-exposure antibiotics.

Partner treatment is part of treatment, not an afterthought: without it, reinfection is routine. Many services offer expedited partner therapy or partner notification support.

Test of cure matters for gonorrhoea (because of resistance) and in pregnancy.

Prevention

• Vaccination: HPV (highly effective, ideally before sexual debut, now often gender-neutral programmes), hepatitis B, hepatitis A for specific groups, and mpox for those at risk.
• Condoms, which remain the only method that reduces most STIs and pregnancy simultaneously.
• HIV PrEP (Chapter 30).
• Doxy-PEP: a single dose of doxycycline within 72 hours after condomless sex substantially reduces subsequent syphilis, chlamydia, and, less reliably, gonorrhoea in trials among men who have sex with men and transgender women. It is now recommended for specific groups, with legitimate concerns about selecting for antimicrobial resistance in other organisms, which is being monitored.
• Routine screening: annual chlamydia and gonorrhoea testing for sexually active young people and more frequent testing for those with higher exposure; universal syphilis, HIV, and hepatitis B testing in pregnancy.

What treatment costs

• Doxycycline: nausea, photosensitivity, oesophageal irritation if taken lying down. Not in pregnancy.
• Ceftriaxone: an injection, which is a real adherence barrier, plus injection site pain.
• Penicillin for syphilis: the Jarisch-Herxheimer reaction, a few hours of fever, chills, and worsening symptoms as large numbers of organisms die and release their contents. It is expected, self-limiting, and alarming if nobody warned the patient. Penicillin allergy requires desensitisation in pregnancy, because no adequately proven alternative exists for preventing congenital syphilis.
• Metronidazole: metallic taste, nausea, and an interaction with alcohol.
• Aciclovir: very well tolerated; suppressive therapy is a daily tablet.

What the person can do

• Test regularly if you have new or multiple partners, and test between partners rather than after symptoms. Most STIs are found by looking, not by feeling.
• Test in pregnancy, and make sure syphilis testing happened. In several countries the congenital syphilis resurgence has been traced to missed or late antenatal testing.
• Get the HPV vaccine if eligible, regardless of gender.
• Use condoms, and understand what they do and do not cover.
• Tell partners. Anonymous partner notification services exist in many places precisely because this conversation is hard, and untreated partners are the main route to reinfection.
• Finish the treatment and attend follow-up.
• Ask about the right test for the right site. Throat and rectal infections are missed by urine testing alone, and this is a common gap in care.

Living with it

Stigma is the defining clinical problem. It delays testing, deters disclosure, and produces distress out of proportion to the medical severity, particularly for herpes, which is common, usually mild, and carries a social weight far exceeding its clinical one. Around two-thirds of the world's population under 50 carries HSV-1, and a large share of adults carry HSV-2, most without knowing.

The practical consequence for clinicians and for anyone giving advice: normalising testing does more for population health than emphasising risk. Services that offer routine opt-out testing, online self-sampling kits, and rapid results consistently find more infections earlier than those relying on people to present with symptoms and a confession.

What's next

• Gonorrhoea antibiotics: new agents such as zoliflodacin and gepotidacin have reported positive phase 3 results, the first genuinely new options in decades.
• A gonorrhoea vaccine: meningococcal B vaccines provide partial cross-protection against gonorrhoea (the organisms are close relatives), and targeted programmes are being trialled.
• Herpes vaccines and therapeutic approaches, including gene editing aimed at latent virus, in early research.
• Cervical cancer elimination, with WHO's 90-70-90 targets (90 percent of girls vaccinated, 70 percent of women screened twice by 45, 90 percent of those with disease treated), which is achievable and off track in most of the world.
• Point-of-care testing giving results within a consultation, which would end the problem of people who never return for results or treatment.

Sources and notes

Global STI incidence (over 1 million curable infections acquired daily) is a WHO estimate. Asymptomatic proportions for chlamydia and gonorrhoea are from surveillance and screening studies and vary by population and site tested. Congenital syphilis increases: US CDC surveillance reports and European equivalents. Gonorrhoea resistance history and current status: WHO Gonococcal Antimicrobial Surveillance Programme. Doxy-PEP: Luetkemeyer et al., NEJM, 2023. Meningococcal B vaccine cross-protection against gonorrhoea: Petousis-Harris et al., The Lancet, 2017, and subsequent studies. HSV prevalence estimates: WHO, 2020. Tuskegee: the Final Report of the Tuskegee Syphilis Study Ad Hoc Advisory Panel, 1973, and subsequent historical analysis. Syphilis origin debate: paleopathological literature, unresolved.

Open questions. The origin of syphilis in Europe remains disputed. Whether doxy-PEP will drive clinically significant antimicrobial resistance is being actively monitored. Why syphilis has resurged so sharply in high-income countries is only partly explained.

Next: the slow-motion emergency underneath all the infection chapters. 👉