Chlamydia spreads through sexual contact, specifically 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, and most people who have it don’t know because it rarely causes obvious symptoms.
How Chlamydia Spreads During Sex
The bacteria 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 unprotected sex. Vaginal and anal sex carry the highest transmission risk, but oral sex can also spread the infection to the throat.
Rectal infections can happen in two ways: through receptive anal sex, or by the bacteria spreading from another infected site like the vagina. This means you can develop a rectal infection even without having had anal sex. The bacteria can also reach the eyes through direct contact with infected genital or urinary fluids, typically by touching your eyes with contaminated hands.
What Happens Inside Your Body
Once the bacteria reach a mucous membrane, they work fast. Chlamydia exists in two forms: a tough, dormant form that survives outside cells and initiates infection, and a fragile, active form that multiplies inside your cells. Within minutes of attaching to a cell, the bacteria inject proteins that manipulate the cell’s internal scaffolding, allowing the bacteria to slip inside.
Once inside, the bacteria create a protective bubble called an “inclusion” and begin replicating by hijacking nutrients from the host cell. Eventually, the newly formed bacteria convert back into their infectious form and break out of the cell to infect neighboring tissue. This cycle is why untreated chlamydia can gradually spread and cause damage over weeks or months, even when you feel perfectly fine.
Why Most People Don’t Know They Have It
Chlamydia is often called a “silent” infection. The majority of people with chlamydia have no symptoms at all, which is exactly why it spreads so easily. You can carry and transmit the bacteria for months without any sign that something is wrong.
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, symptoms can include discharge from the penis, burning with urination, or pain and swelling in one or both testicles. Rectal infections may cause discharge, pain, or bleeding, though they’re often symptomless too.
Ways You Cannot Get Chlamydia
Chlamydia bacteria cannot survive outside the human body. You cannot get chlamydia from toilet seats, bath towels, bed linens, swimming pools, or hot tubs. The bacteria require the warm, moist environment of human mucous membranes to survive, and chlorinated water and open air kill them quickly. Casual contact like hugging, shaking hands, sharing food, or sitting next to someone poses zero risk.
Passing Chlamydia to a Newborn
A pregnant person with an untreated chlamydia infection can pass the bacteria to their baby during vaginal delivery. This can cause eye infections (conjunctivitis) or pneumonia in the newborn, and has been linked to preterm delivery and low birth weight. Routine prenatal screening catches most cases before delivery, and treatment during pregnancy prevents transmission to the baby.
When Testing Becomes Accurate
If you’ve had a possible exposure, testing too early can give you a false negative. A urine test or swab (vaginal, rectal, or throat, depending on the type of contact) will detect most infections after one week. Waiting two weeks catches nearly all cases. This window matters: getting tested the day after exposure is unlikely to give you a reliable result.
Reinfection Is Common
Having chlamydia once does not protect you from getting it again. In a study of adolescents and young adults who were treated for chlamydia and retested within the recommended window of one to twelve months, 22% tested positive again. That’s roughly one in five people picking up a new infection shortly after clearing the first one.
This happens for a straightforward reason: treatment cures the infection in your body, but it doesn’t create immunity. If your sexual partner wasn’t treated at the same time, or if you have a new partner who carries the bacteria, you’re just as vulnerable as before. Current guidelines recommend retesting about three months after completing treatment to catch reinfections early.
Reducing Your Risk
Latex condoms, used consistently and correctly, reduce the risk of chlamydia transmission. They aren’t perfect, since any skin-to-skin contact outside the area covered by the condom still carries some risk, but they provide meaningful protection. Internal (female) condoms and dental dams offer similar barrier protection for vaginal and oral sex.
Regular screening is the other major line of defense, especially since most infections are symptomless. Annual testing is generally recommended for sexually active women under 25, anyone with new or multiple partners, and men who have sex with men. If you test positive, your recent sexual partners need to be notified and treated to break the cycle of transmission.