15.13 — Sexual Health and Sexually Transmitted Infections

Most sexually transmitted infections cause no symptoms at all in most people who have them. Around 70 percent of women and 50 percent of men with chlamydia have none.

Which is the central fact of this subject. The infections spread precisely because they are silent, the damage accumulates without warning, and testing rather than symptoms is what finds them.

And the second central fact is that essentially all of them are either curable or controllable, and several are now preventable by vaccine. This is one of the more optimistic areas of medicine, and the main obstacle to acting on it is embarrassment.

The bacterial infections — curable

Chlamydia

The commonest bacterial STI, and the one with the largest cumulative consequence.

Peak incidence in the under-25s.

Symptoms, when present: discharge, pain on passing urine, bleeding between periods or after sex in women, and testicular pain in men.

And the consequence of not treating it is the reason it matters. Ascending infection causes pelvic inflammatory disease (Chapter 15.3), and each episode causes tubal scarring — roughly 12 percent infertility after one episode, 25 percent after two, 50 percent after three — plus a substantially increased risk of ectopic pregnancy and chronic pelvic pain.

In men it can cause epididymitis and, uncommonly, reactive arthritis.

Testing: a urine sample in men, a self-taken vaginal swab in women. Nucleic acid amplification, highly sensitive, and no examination required.

Treatment: doxycycline for 7 days is now first-line, having replaced single-dose azithromycin as resistance and efficacy data changed.

And partner notification is part of the treatment, not an optional extra — otherwise reinfection is near-certain.

Screening programmes in the under-25s exist in many countries, and their value lies entirely in the asymptomatic majority.

Gonorrhoea

Second commonest bacterial STI, and the one to watch for resistance.

Symptoms: thick discharge and pain on passing urine, more often symptomatic in men than in women. Throat and rectal infections are usually asymptomatic, which is why testing at those sites matters in people who have that exposure.

Complications: pelvic inflammatory disease, epididymitis, and rarely disseminated infection with joint and skin involvement.

And in a newborn, gonococcal conjunctivitis can cause blindness within days — which is why eye prophylaxis at birth is routine in many countries and why maternal screening matters.

Resistance is the serious problem. Neisseria gonorrhoeae has developed resistance to every antibiotic class used against it, in sequence — sulfonamides, penicillins, tetracyclines, fluoroquinolones, and now increasingly to cephalosporins.

Treatment is now a single injection of ceftriaxone, at a dose that has been raised repeatedly, and strains resistant to it have been reported. The WHO lists it as a high-priority pathogen for new antibiotic development, and this is one of the clearest live examples of antimicrobial resistance narrowing options (Chapter 17.12).

Test of cure is now standard, which is unusual and reflects the resistance situation.

Syphilis

Caused by a spirochaete, and it is the one that mimics everything — historically called "the great imitator".

Incidence has risen substantially in many countries over the last two decades, after decades of decline.

Four stages:

Primary — a painless ulcer (chancre) at the site of infection, 3 weeks after exposure, healing spontaneously in 3 to 6 weeks.

And painlessness is the diagnostic feature. A painful genital ulcer is usually herpes; a painless one is syphilis until proven otherwise. The chancre heals whether treated or not, which is exactly why it is missed.

Secondary — weeks to months later. A widespread rash characteristically involving the palms and soles, which very few rashes do; fever; lymph node enlargement; mucous patches; and patchy hair loss. Also resolves spontaneously.

Latent — no symptoms, positive tests, lasting years.

Tertiary — in around a third of untreated cases, after 10 to 30 years. Gummas destroying tissue; cardiovascular syphilis affecting the aorta; and neurosyphilis with dementia, personality change, and the characteristic pupil that accommodates but does not react to light.

Congenital syphilis — crossing the placenta and causing stillbirth, or bone, tooth, eye and neurological abnormalities.

And it is entirely preventable by testing and treating the mother (Chapter 15.6). Its persistence anywhere is a failure of antenatal care rather than a medical difficulty.

Treatment remains benzathine penicillin, by injection. There has never been documented penicillin resistance in syphilis, which is remarkable after eighty years of use.

And one specific phenomenon must be warned about: the Jarisch–Herxheimer reaction. Fever, chills and worsening rash within hours of the first dose, caused by the mass death of organisms releasing inflammatory material. It is expected, self-limiting, and alarming if unexplained — and in pregnancy it can precipitate contractions, so treatment is given where monitoring is available.

Trichomoniasis

A protozoan (Chapter 3.7). Frothy discharge with an offensive smell, or no symptoms at all — around half of women and most men are asymptomatic.

Treated with metronidazole, and it increases HIV transmission risk, which is one reason for treating asymptomatic infection.

The viral infections — controllable

Genital herpes

Herpes simplex virus, types 1 and 2. The traditional division — type 1 oral, type 2 genital — no longer holds; a large and rising proportion of genital herpes is type 1, from oral sex.

Primary episode: painful blisters and ulcers, fever, and lymph node enlargement. The first episode is by far the worst.

The virus then travels up the nerve and lies dormant in the sensory ganglion (Chapter 11.3), reactivating intermittently. Recurrences are shorter, milder and become less frequent over years.

Asymptomatic shedding is why it spreads — the virus is released from the skin without any visible lesion, on a proportion of days.

Treatment: antivirals shorten episodes when started early; suppressive daily treatment reduces recurrences by around 70 to 80 percent and reduces transmission to a partner by around 50 percent.

And the psychological impact substantially exceeds the medical one. Herpes is common — a large proportion of adults carry HSV-1 — usually mild, and does not affect fertility or general health. The distress on diagnosis is real and is driven largely by stigma rather than by the illness, and accurate information is a genuine part of the treatment.

The one situation where it is serious: primary infection acquired late in pregnancy, which carries a real risk of neonatal herpes — a severe illness. Caesarean delivery is recommended in that circumstance, and recurrent herpes in a woman who acquired it before pregnancy carries a far lower risk because maternal antibody has crossed the placenta.

HPV

The commonest STI of all — most sexually active people acquire it at some point, and the great majority clear it within 1 to 2 years without ever knowing.

Over 200 types.

Low-risk types (6 and 11) cause genital warts.

High-risk types (16 and 18, plus others) cause essentially all cervical cancer, plus a large share of anal, vulval, vaginal, penile and oropharyngeal cancers.

And persistent infection with a high-risk type over years is what causes cancer, not a transient one — which is why screening looks for persistence and for cell changes rather than for the virus alone.

Prevention is where this becomes a success story (Chapter 13.5).

HPV vaccination before exposure prevents the infection. The Swedish population study found an 88 percent reduction in invasive cervical cancer in those vaccinated before 17.

Combined with screening, the WHO has set an elimination target for cervical cancer, and several countries are on course. Australia may eliminate it as a public health problem within the next decade.

A vaccine that prevents a cancer, given to teenagers, on course to eliminate a disease that kills over 300,000 women a year — it is worth stating plainly, because the vaccine has attracted more misinformation than almost any other.

Cervical screening — now primarily HPV testing rather than cytology, because it is more sensitive and allows longer intervals in those who test negative (Chapter 15.3).

HIV

Chapter 17.9 covers it in full. The essential points for sexual health:

Testing is the entry point to everything. A substantial proportion of transmission comes from people who do not know they are infected.

Treatment is effective and simple — usually one tablet a day, with a near-normal life expectancy.

And the single most important fact: U=U — undetectable equals untransmittable.

A person on effective treatment with an undetectable viral load cannot transmit HIV sexually. This is established by large studies with zero transmissions, it is endorsed by every major health body, and it has transformed both the medical and the social meaning of the diagnosis.

PrEP — pre-exposure prophylaxis — antiretroviral medication taken by an HIV-negative person, reducing sexual transmission risk by around 99 percent when taken as prescribed.

PEP — post-exposure prophylaxis — started within 72 hours of a possible exposure, ideally within hours, and taken for 28 days.

Hepatitis B

Sexually transmissible, and preventable by vaccine (Chapter 17.10).

Vaccination is universal in infancy in most countries, and it is recommended for anyone at risk who was not vaccinated.

Mpox

Included because the 2022 global outbreak spread predominantly through sexual contact, mainly among men who have sex with men, and it demonstrated something worth learning: rapid, non-stigmatising engagement with the affected community, combined with targeted vaccination, brought the outbreak under control quickly.

Testing

And the practical points matter more than the list.

Window periods — the time after exposure before a test becomes reliable.

Chlamydia and gonorrhoea: 2 weeks. HIV: 45 days for a fourth-generation test, 90 days to be definitive. Syphilis: 12 weeks.

Testing too early gives false reassurance, which is the commonest error.

Site-specific testing. Throat and rectal infections are not detected by a urine sample. Anyone with those exposures needs swabs from those sites, and this is frequently omitted.

Self-testing and postal kits are now widely available and have substantially increased testing rates, particularly among people who would not attend a clinic.

Who should test: anyone with symptoms; anyone with a new partner; anyone whose partner has tested positive; anyone in a group with higher prevalence, at regular intervals; and everyone in pregnancy.

Prevention

Condoms — highly effective against HIV, gonorrhoea, chlamydia and trichomoniasis. Less effective against herpes and HPV, because those transmit from skin not covered by the condom.

Vaccination — HPV and hepatitis B, and hepatitis A and mpox for specific groups.

PrEP for HIV.

Regular testing.

Partner notification — and it is worth defending, because it feels intrusive. It prevents reinfection, it prevents onward transmission, and it can be done anonymously through clinic services.

Doxycycline post-exposure prophylaxis (doxy-PEP) — a single dose after condomless sex — reduces bacterial STIs substantially in high-risk groups, and its use is being expanded while resistance implications are monitored. A genuinely new intervention, and one whose long-term antibiotic resistance consequences are not yet clear.

Talking about it

A short section, because it is where the medicine actually fails.

Shame is the main obstacle to sexual health, and it operates in both directions: people do not test, and clinicians do not ask.

What helps:

Sexual health services are confidential, and in most countries this is legally protected and separate from other medical records.

They are free in many health systems, and open-access without referral.

Nothing you say will surprise anyone working there.

And STIs are not a moral event. They are infections, transmitted by an extremely common human activity, most of them curable and all of them manageable. Framing them otherwise is precisely what keeps people from being tested — which is what allows them to spread.

For young people specifically, comprehensive sex education is associated with later first intercourse, higher contraceptive use and lower rates of infection and unintended pregnancy, and abstinence-only programmes are consistently found not to delay sex and to reduce contraceptive use when it happens. That is what the evidence shows, and it is worth stating.

What Part 16 does next

The body has now been described end to end — every system, in health. From here the volume turns to disease.

Part 16 is the framework: what "disease" actually means, how cells are injured and die, how a doctor reaches a diagnosis, how to read your own blood report line by line, and what each kind of scan can and cannot see.

And it is the last Part written at full anatomical depth. From Part 17 onward the emphasis shifts — the same accuracy, told more briskly, and consistently ending on what can be prevented, what can be treated, and what you control.