Chlamydia spreads through sexual contact, primarily vaginal, anal, or oral sex without a condom with someone who has the infection. It can also pass from a mother to her baby during childbirth and, less commonly, from the genitals to the eyes via contaminated hands. What makes chlamydia especially easy to pass is that most people who carry it have no idea they’re infected.
Vaginal, Anal, and Oral Sex
The bacteria behind chlamydia, called Chlamydia trachomatis, targets the moist mucous membranes lining the genitals, rectum, and throat. It latches onto cells in these areas, works its way inside them, and begins replicating. Any unprotected sexual contact that brings these surfaces together can transfer the bacteria from one person to another.
Vaginal sex is the most common route. The per-partnership transmission probability from an infected man to a woman is roughly 32 to 35%, while female-to-male transmission per partnership is lower, though estimates vary widely. Per single act of sex, the risk ranges from about 4% to 15%, depending on the direction of transmission and the study. These aren’t small numbers, especially over the course of a relationship with multiple encounters.
Rectal infections happen through receptive anal sex, but they can also develop when the bacteria spreads from a nearby infected site, like the vagina. Oral sex carries a lower risk, though it can still lead to a throat infection that often produces no symptoms at all.
Why Most People Don’t Know They Have It
About 75% of women and 50% of men with chlamydia have no symptoms. This is the single biggest reason the infection spreads so easily. Someone can carry and transmit the bacteria for weeks or months without any sign that something is wrong. The incubation period is typically 7 to 21 days, but it can stretch to several months, and a person remains contagious the entire time they’re untreated.
Because the majority of cases go undiagnosed and unreported, chlamydia circulates quietly through sexual networks. A person who feels perfectly healthy can pass the infection to every new partner until they happen to get tested.
The Reinfection Cycle Between Partners
Even after successful treatment, getting chlamydia again is common. In one modeling study, about 16% of women were reinfected within 12 months of treatment. A major driver is what’s sometimes called the “ping-pong” effect: one partner gets treated, but the other doesn’t, and the untreated partner passes the bacteria right back.
Research suggests that at least 30% of current sex partners need to be notified and treated to meaningfully counterbalance this reinfection cycle. This is why clinics typically recommend that all recent sexual partners get treated at the same time, even if they have no symptoms. Without that step, treatment for one person can simply reset the clock rather than eliminate the infection from the partnership.
Transmission During Childbirth
A pregnant person with an untreated chlamydial infection can pass the bacteria to their baby during vaginal delivery. As the newborn moves through the birth canal, the bacteria can infect the eyes and lungs. Eye symptoms, including discharge, redness, and swollen eyelids, typically appear within the first 15 days of life. Lung infections can develop into pneumonia.
Chlamydia during pregnancy also raises the risk of preterm delivery. One population-based study found that infected mothers were more than four times as likely to deliver before 32 weeks of gestation. Roughly 15% of very early preterm births in that study were attributable to the infection. This is why routine screening during pregnancy is standard practice, with retesting recommended three months after treatment.
Eye Infections From Touch
Chlamydia can infect the eyes when contaminated genital fluids reach the conjunctiva, the thin membrane covering the white of the eye. This usually happens through hand-to-eye contact: touching infected genitals, then rubbing your eyes without washing your hands. It can also spread through shared towels or bedding that carry fresh secretions.
The resulting condition, called inclusion conjunctivitis in adults, typically affects one eye. It causes a mucus-like discharge, redness, a gritty foreign-body sensation, sensitivity to light, and sometimes swollen lymph nodes near the eye. It’s not the same as the trachoma that causes blindness in parts of the developing world (that involves different strains of the same bacteria), but it still requires treatment.
What Doesn’t Spread Chlamydia
Chlamydia trachomatis cannot survive long outside the human body. You won’t get it from toilet seats, swimming pools, hot tubs, shared utensils, or casual contact like hugging or shaking hands. The bacteria requires direct contact with mucous membranes or bodily fluids to establish an infection. Sitting where someone else sat, or sharing a drink, poses essentially zero risk.
How Long You Stay Contagious
Without treatment, a person with chlamydia remains infectious indefinitely. The bacteria doesn’t clear on its own in a predictable timeframe. Once treatment begins, most guidelines consider a person no longer contagious after completing the full course and waiting at least seven days. Retesting is recommended about three months after treatment, not because the medication failed, but because reinfection from an untreated partner or a new exposure is so common.
Condoms used consistently and correctly during vaginal, anal, and oral sex significantly reduce the risk of transmission, though they don’t eliminate it entirely since the bacteria can be present on surfaces not fully covered. Regular screening is the other key tool, particularly for sexually active women under 25, who carry the highest burden of undiagnosed infections.