Chlamydia spreads through vaginal, anal, or oral sex without a condom with someone who has the infection. It’s the most commonly reported bacterial sexually transmitted infection in the United States, with over 1.5 million cases reported in 2024 alone. Because the majority of infected people have no symptoms, many pass it to partners without ever knowing they carry it.
Sexual Contact Is the Primary Route
The bacterium that causes chlamydia, called Chlamydia trachomatis, lives in the mucous membranes of the genitals, rectum, and throat. It passes from one person to another through direct contact with these tissues during sex. That means vaginal intercourse, anal intercourse, and oral sex can all transmit the infection. Penetration doesn’t need to be deep or prolonged. Any contact between infected mucous membranes and a partner’s genital, rectal, or throat tissue creates an opportunity for transmission.
Rectal infections can happen in two ways: through receptive anal sex, or when the bacteria spreads from a nearby infected site like the vagina. This means someone can develop a rectal infection even without having had anal sex.
What Happens Inside the Body
Once the bacteria reach a mucous membrane, they attach to cells in the inner lining of the tissue. Within about two hours of entering a cell, the bacteria shift into a growth phase, dividing rapidly and multiplying in number. The infected cells eventually burst, releasing new bacteria that go on to infect neighboring cells. This cycle can continue for weeks or months without producing noticeable symptoms, which is a major reason the infection spreads so easily.
After exposure, chlamydia typically becomes detectable within one week. Testing at two weeks after exposure catches nearly all infections.
Most Infections Produce No Symptoms
This is the single most important thing to understand about chlamydia transmission: the majority of people who have it don’t know. At least 70% of women and 50% of men with a genital chlamydia infection have no symptoms at the time of diagnosis. Seven out of ten women with a cervical infection have no symptoms, or only mild ones that wouldn’t prompt a doctor visit.
That means a partner who looks and feels perfectly healthy can still transmit the infection. It also means you can carry chlamydia for months without realizing it, potentially passing it to multiple partners during that time. This is why routine screening matters so much, especially for sexually active people under 25, who account for a disproportionate share of cases.
Less Common Ways It Spreads
Chlamydia can infect the eyes through hand-to-eye contact with infected genital secretions. This leads to a condition called chlamydial conjunctivitis, which causes redness, mucous discharge, crusty eyelashes, swollen lids, tearing, and sometimes blurred vision. Symptoms can be mild and drag on for weeks to months before someone seeks care, or they can appear more suddenly with heavier discharge. It typically affects one eye but can involve both.
An infected mother can also pass chlamydia to her baby during vaginal delivery, which is why prenatal screening is standard practice.
How You Cannot Get Chlamydia
The bacteria that cause chlamydia cannot survive long outside the human body. You won’t get it from a toilet seat, a swimming pool, sharing food, or touching a doorknob. The bacteria need the warm, moist environment of mucous membranes to stay alive. While there’s a very low theoretical possibility of transmission through contaminated fabrics like sheets or towels, this is not considered a meaningful route of infection in practice. Chlamydia is, for all practical purposes, only transmitted through sexual contact or from mother to child during birth.
Reinfection After Treatment
Getting treated for chlamydia does not make you immune. You can catch it again immediately after finishing treatment if you have sex with an untreated partner. Research shows that as many as 20% of women test positive for chlamydia again within six months of their initial diagnosis and treatment. The most common reason is straightforward: their partner never got treated, and the infection passed right back.
This is why health guidelines emphasize that all sexual partners from the previous 60 days need treatment, not just testing. In many cases, a partner can receive treatment at the same visit or even through medication delivered by the diagnosed person, depending on local policies. If the most recent sexual contact was more than two months before diagnosis, that partner should still be offered testing and treatment. Waiting for a partner’s test results before treating them isn’t recommended, because the delay creates more time for the infection to spread.
Reducing Your Risk
Condoms, when used consistently and correctly during vaginal, anal, and oral sex, significantly reduce the chance of transmission. They don’t eliminate risk entirely because chlamydia can infect areas a condom doesn’t cover, but they remain the most effective barrier method available.
Regular screening is the other critical layer of protection. Because chlamydia so often produces no symptoms, testing is the only reliable way to catch an infection early. If you’re sexually active with new or multiple partners, periodic screening lets you identify and treat infections before they cause complications like pelvic inflammatory disease in women or spread to additional partners. A simple urine test or swab is all that’s needed, and treatment with antibiotics typically clears the infection quickly.