Chlamydia spreads through vaginal, anal, or oral sex without a condom with someone who has the infection. It’s the most common bacterial sexually transmitted infection in the United States, and most people who have it don’t know because they never develop symptoms. That combination of easy transmission and silent infection is what makes it so widespread.
Sexual Contact Is the Primary Route
The bacterium that causes chlamydia lives in the mucous membranes of the genitals, rectum, and throat. Any unprotected sexual contact that brings these tissues together can pass the infection. Vaginal and anal sex carry the highest risk. Oral sex can also transmit it, though the risk is lower and less well studied. You can get a chlamydia infection in your throat from giving oral sex to an infected partner, or get a genital infection from receiving oral sex from someone with the bacteria in their throat.
Rectal infections happen through receptive anal sex, but they can also develop when the bacteria spreads from a vaginal infection to the rectum without any anal contact at all. This means you can have chlamydia in more than one location at the same time without realizing it.
The estimated transmission probability is around 4.5% per single unprotected sexual encounter. That may sound low, but the risk accumulates quickly over multiple encounters with an infected partner, and the math changes considerably when you factor in how many people carry the infection unknowingly.
Why Most People Don’t Know They Have It
About 75% of women and 50% of men with chlamydia have no symptoms whatsoever. This is the main reason the infection spreads so efficiently. Someone can carry and transmit chlamydia for weeks or months without any sign that something is wrong, passing it to new partners who also remain unaware.
When symptoms do appear, they typically show up one to three weeks after exposure. In women, this might look like unusual vaginal discharge, burning during urination, or bleeding between periods. In men, it often presents as discharge from the penis, burning when urinating, or pain and swelling in one testicle. Rectal infections can cause discharge, pain, or bleeding from the rectum. Throat infections rarely cause noticeable symptoms at all.
What Happens Inside the Body
Chlamydia is caused by a bacterium that can’t survive on its own. It needs to get inside your cells to access their energy supply. Once the bacteria land on a mucous membrane, tiny spore-like particles called elementary bodies attach to cells and get pulled inside. Once there, they transform into an active form that hijacks the cell’s resources to multiply. Eventually the host cell fills up, bursts open, and releases a wave of new infectious particles that latch onto neighboring cells. This cycle repeats, spreading the infection across the tissue.
This is why untreated chlamydia can cause serious damage over time. In women, the infection can climb from the cervix into the uterus and fallopian tubes, leading to pelvic inflammatory disease and potentially affecting fertility. In men, it can spread to the tube that carries sperm and cause pain and swelling, though lasting complications are less common.
Ways You Cannot Catch Chlamydia
The bacteria survive poorly outside the human body. On a dry surface like glass, it lasts about 30 minutes. You cannot get chlamydia from a toilet seat, swimming pool, shared towel, or casual contact like hugging or shaking hands. It requires direct contact with infected mucous membranes or the fluids they produce.
There is one non-sexual exception: you can transfer the bacteria from an infected genital area to your own eyes on your fingers. This hand-to-eye spread of infected secretions causes a form of conjunctivitis that needs treatment. A pregnant person with chlamydia can also pass the infection to their baby during vaginal delivery, which can cause eye infections or pneumonia in the newborn.
When Testing Can Detect It
If you think you’ve been exposed, testing too early can produce a false negative. The bacteria need time to establish an infection large enough for a test to pick up. One week after exposure catches most infections. Waiting two weeks catches nearly all of them. Testing uses either a urine sample or a swab from the vagina, rectum, or throat, depending on where the exposure occurred. If you’ve had both oral and genital contact with a partner, it’s worth testing more than one site, since a standard urine test won’t detect a throat or rectal infection.
How to Reduce Your Risk
Consistent and correct condom use reduces the risk of chlamydia transmission significantly, though no method is 100% effective. Because the infection is so often asymptomatic, regular screening is just as important as barrier protection. The CDC recommends annual screening for all sexually active women under 25, and for anyone with new or multiple partners. If you test positive, treatment is straightforward with antibiotics, and sexual partners from the previous 60 days should also be tested and treated to prevent reinfection. Retesting about three months after treatment is recommended, since reinfection rates are high.