The infection
Chlamydia is caused by the bacterium Chlamydia trachomatis, which infects the genital, anal, throat and eye mucous membranes. It is the most frequently reported bacterial STI in high-income countries, peaking at 15-25. Its defining feature is silence: most infected people have no idea they are carrying it, continue to pass it on, and only discover it through a complication or routine screening.
Where does France stand?
In 2024, about 61,100 people were diagnosed with chlamydia and treated in the private sector, according to national health insurance data, and a further 22,200 infections were diagnosed in CeGIDD, the free screening centres (SpF, 2025). The number of people diagnosed and treated in the private sector rose by 15% between 2022 and 2024, and 56% of these diagnoses concern men (Santé publique France, 2025).
The rise has accelerated among young people: between 2023 and 2024, the incidence rate increased by 13% in women aged 15 to 25 and by 21% in men of the same age (SpF, 2025). These figures need one caveat: testing also rose, by 30% between 2022 and 2024, with 3.4 million people tested in 2024 (SpF, 2025). The more you look for the infection, the more you find it, and that is exactly what is needed: an infection that is found can be treated.
Key facts
| Organism | Bacterium Chlamydia trachomatis |
|---|---|
| Incubation | 1 to 3 weeks, often with no sign at all |
| Asymptomatic cases | About half of cases, in women and men alike (Santé publique France, 2025) |
| Diagnoses in France in 2024 | About 61,100 people diagnosed and treated in the private sector, plus 22,200 diagnoses in CeGIDD (SpF, 2025) |
| Reliable testing from | About two weeks after exposure |
| Sample | Self-taken vaginal swab or first-void urine; throat and rectal swabs if exposed |
| Curable | Yes, with a one-week course of antibiotics |
| Vaccine | None |
How it is transmitted
It passes through unprotected vaginal, anal and oral sex, by contact between mucous membranes. Semen is not required: contact between mucous membranes is enough. It can also be passed from mother to baby during birth,with a risk of conjunctivitis and pneumonia in the newborn (WHO, 2025). Sharing unwashed or uncovered sex toys can also transmit the infection.
Symptoms
Where symptoms appear, they do so one to three weeks after exposure: clear or cloudy discharge, burning on passing urine, pelvic pain, bleeding between periods or after sex in women; urethral discharge and discomfort passing urine in men. Rectal and throat infections are almost always silent. But the essential point lies elsewhere: in about half of cases there is no sign whatsoever, and the absence of symptoms must never be read as the absence of infection. Estimates vary between sources: Santé publique France puts it at half of cases, whereas the ECDC, the European agency, states that at least 70% of women and about half of men (with estimates ranging from 25 to 100%) have no symptoms when the infection is genital (ECDC, 2024).
Testing
Testing uses a nucleic acid amplification test (PCR), which is highly accurate. For women, a self-taken vaginal swab performs as well as a clinician-taken sample (WHO, 2025) and is more comfortable. For men, first-void urine is sufficient — wait at least an hour after last passing urine. If you have anal or oral sex, ask specifically for rectal and throat swabs: a urine test alone misses those sites. The result is reliable about two weeks after exposure.
Getting tested in France
Since September 2024, the "Mon test IST" scheme has allowed anyone to be tested at any medical biology laboratory, without a prescription and without an appointment. It covers five infections: HIV, hepatitis B, syphilis, gonorrhoea and chlamydia. Before age 26, national health insurance covers these tests at 100%, with nothing to pay up front. From age 26, only the HIV test is covered at 100%; the other four are reimbursed 60% by national health insurance and 40% by supplementary health insurance, with flat-rate contributions and deductibles still applying. For free and anonymous testing, CeGIDD remain the answer.
Another route exists for 18-25 year-olds: a self-sampling kit sent to the home, ordered on the site mon-test-ist.ameli.fr and covered at 100%, which tests for chlamydia and gonorrhoea. Open to young women since 1 July 2025, it has been extended since 29 May 2026 to men aged 18 to 25, as well as to transgender, queer or non-binary people of the same age group.
Treatment
The first-line treatment is doxycycline, 100 mg twice daily for seven days, which outperforms single-dose azithromycin, particularly for rectal infection. Azithromycin remains an option in pregnancy or where the course cannot be followed. Avoid sex for the full seven days of treatment and until partners have been treated, or reinfection is immediate.
National health insurance points out that the infection is easily treated with an antibiotic. A pregnant woman receives treatment suited to pregnancy, and a second antibiotic (a ceftriaxone injection) is added if there is a simultaneous gonorrhoea infection. Lymphogranuloma venereum requires a longer course, of three to four weeks. A repeat test to check the cure, three to five weeks after the end of treatment, is only useful in certain situations: pregnancy, persistent symptoms or difficulty following the treatment.
If left untreated
Untreated, chlamydia ascends to the uterus and tubes and causes salpingitis, often silent too. The consequences are chronic pelvic pain, ectopic pregnancy and tubal infertility (WHO, 2025): it is the leading preventable cause of infertility in women. The real risk of long-term damage is nonetheless poorly understood: the ECDC notes that recent data suggest it may be lower than was thought, and cites a British study estimating about 171 cases of pelvic inflammatory disease per 1,000 infections in women aged 16 to 44 (ECDC, 2024). In men it can cause epididymitis. Particular strains cause lymphogranuloma venereum (Santé publique France, 2025), with severe rectal disease.
Prevention
- Condoms, including for oral and anal sex, remain the main protection.
- Annual screening is recommended for sexually active women under 25, even without symptoms and even with a single partner; for men, it depends on risk factors.
- Test at every change of partner, and before stopping condoms in a new relationship.
- Clean sex toys between uses and between partners, or cover them with a condom.
After a diagnosis
- Notify every partner you have had sex with in the last twelve months, although US guidelines specify the last two months (CDC, 2021); if there has been no partner in that period, notify the most recent partner (CDC, 2021).
- Partners must be treated even if their test is negative or they have no symptoms.
- Retest about three months after treatment (CDC, 2021): reinfection is very common, far more so than treatment failure.
- Use the diagnosis as a prompt for a full screen: gonorrhoea, syphilis, HIV and hepatitis.
Worth remembering
A negative test taken too early means nothing. If the exposure was less than two weeks ago, repeat the test rather than relying on the first result.
A little history
Chlamydia trachomatis was first known through another disease it causes: trachoma, the blinding eye infection described in the Ebers papyrus in Egypt 3,500 years ago. In 1907, in Java, Ludwig Halberstaedter and Stanislaus von Prowazek observed characteristic "inclusions" in conjunctival cells and proposed the name Chlamydozoa, from the Greek for "cloak". For half a century it was taken for a virus: it does not grow on ordinary culture media.
The trachoma agent was isolated in 1957 by the Chinese microbiologist Tang Fei-fan, in embryonated eggs. In the 1960's it was established that it is a bacterium after all — but one that has to live inside cells — and it was understood that it also causes the "non-gonococcal" urethritis for which no microbe had been found. Diagnosis long remained difficult, confined to laboratories equipped for cell culture.
Everything changed in the 1990's with nucleic acid amplification tests, which can be run on a simple urine sample or a self-taken swab: the scale of a mostly silent infection came to light, and screening programmes for young adults were set up — England launched its own in 2003. Single-dose azithromycin, which arrived in the 1990's, and doxycycline remain the two reference treatments.
How this differs between countries
What differs between countries is neither the organism nor the treatment, but how hard anyone looks for it.
- Organised screening or none. Several European and English-speaking countries recommend annual testing for sexually active people under 25, sometimes with kits posted to the home. Elsewhere nothing is offered without symptoms.
- Syndromic management. Across much of sub-Saharan Africa and South Asia, laboratory diagnosis is not routinely available: treatment follows standardised algorithms based on symptoms. That works for the symptomatic person but misses all the silent infections.
- First-line antibiotic. Seven days of doxycycline has replaced single-dose azithromycin in most recent guidance, but national protocols have not all been updated.
- Cost. Testing is free in dedicated services in many countries and entirely self-funded in others.
- Extragenital swabs. Routine throat and rectal testing is recommended in some countries and absent in others, even though those sites are almost always symptomless.