Chlamydia: the most common STI you've never noticed

Chlamydia's defining feature is how quietly it operates. It's the most commonly diagnosed bacterial STI in most countries, it's easily transmitted, and the large majority of people who have it experience no symptoms whatsoever. That combination — common, silent, and easily spread — is exactly why it's so widespread, and why "I feel fine" tells you nothing about whether you have it. The only way to know is to test.

The good news, stated up front: chlamydia is completely curable with antibiotics, and testing is quick and simple. The whole reason it matters is that catching it early prevents the complications that untreated chlamydia can cause over time.

What it is and how it spreads

Chlamydia is caused by the bacterium Chlamydia trachomatis. It's transmitted through vaginal, anal, and oral sex with someone who has it, and it can infect the genitals, rectum, throat, and eyes. It spreads through the exchange of infected fluids and contact with infected mucous membranes — not through casual contact, toilet seats, sharing cups, or hugging. Penetrative sex isn't required for transmission; genital contact can be enough.

It can also pass from a pregnant person to their baby during birth, which is one reason antenatal screening includes it in many places.

Why symptoms don't help you

The majority of people with chlamydia — a large proportion of those with vulvas and a substantial share of those with penises — have no symptoms at all. When symptoms do appear, they usually show up 1–3 weeks after exposure and can include:

• Unusual discharge from the vagina, penis, or rectum
• Pain or burning when urinating
• Pain during sex, or bleeding between periods or after sex (in people with a uterus)
• Testicular pain or swelling (in people with testicles)
• Rectal pain, discharge, or bleeding (with rectal infection)
• A sore throat (with throat infection, though this is usually symptomless too)

Because symptoms are usually absent or mild enough to ignore, most infections are found through testing rather than because someone felt unwell. This is the core reason routine STI screening exists: you cannot rely on your body to tell you.

Testing

Testing for chlamydia is straightforward and non-invasive. It's usually a urine sample (for genital infection) or a swab — which can often be self-taken — of the vagina, rectum, or throat depending on the type of sex you've had. There are no needles involved for a standard chlamydia test.

Chlamydia becomes detectable about 1–2 weeks after exposure, so testing too soon after a specific worry can miss a recent infection. If you're testing after a particular encounter, around two weeks is the usual guidance. If you're testing as routine screening without a specific worry, any time is fine. At-home test kits are available in many places and are a convenient option (the at-home STI test kits guide covers these). The when-to-get-tested guide covers timing in detail.

Treatment

Chlamydia is cured with antibiotics — typically a short course (commonly doxycycline over several days, or in some cases a single dose of azithromycin, depending on current guidance and the site of infection). Treatment is highly effective. A few important points:

• Complete the full course as prescribed, even though you'll likely feel no different (since you probably had no symptoms to begin with).
• Avoid sex until treatment is complete — usually for seven days after a single-dose treatment, or until a multi-day course is finished — to avoid passing it on or getting reinfected.
• Partners need treating too. Recent sexual partners should be tested and treated, even if they have no symptoms, both for their health and to prevent reinfecting you. Many clinics help with anonymous partner notification.
• Retesting is sometimes recommended a few months later, as reinfection is common — not because treatment fails, but because of ongoing exposure.

Why untreated chlamydia matters

The reason chlamydia is worth finding despite being usually symptomless is the damage it can do over time if left untreated. In people with a uterus, it can spread to the reproductive organs and cause pelvic inflammatory disease (PID), which can lead to chronic pain, ectopic pregnancy, and infertility. In people with testicles, it can cause epididymitis (painful inflammation) and, rarely, fertility problems. Untreated infection also increases susceptibility to HIV.

These complications develop silently, over months or years, from an infection that never announced itself. That's the entire case for testing: the infection is easy to cure when caught, and the consequences of not catching it can be serious and permanent.

Prevention

Condoms significantly reduce chlamydia transmission and are the main prevention method (though they don't eliminate risk entirely, since transmission can occur through contact not covered by a condom). Regular testing — especially with new or multiple partners — catches infections early. And testing between partners, along with open conversations about status, reduces spread. The condoms guide and the how-to-bring-up-STI-status guide cover the practical side.

The overall message on chlamydia is reassuring once you know it: extremely common, usually silent, simple to test for, and completely curable — as long as you actually test, because it will not tell you it's there.