Chlamydia spreads through vaginal, anal, or oral sex with an infected person. It’s the most commonly reported bacterial sexually transmitted infection in the United States, contributing to over 2.2 million combined STI cases reported in 2024. What makes chlamydia especially easy to catch is that most people who have it don’t know they do, so it passes silently between partners.
Sexual Contact Is the Primary Route
The bacterium that causes chlamydia infects the mucous membranes of the body, the moist tissue lining the genitals, rectum, and throat. Any unprotected sexual contact that brings these surfaces together can transmit the infection. That includes vaginal sex, anal sex, and oral sex. You don’t need to have penetrative sex for transmission to occur; genital-to-genital contact can be enough.
Rectal infections can happen through receptive anal sex, but they can also develop when the bacteria spreads from an existing vaginal infection. Similarly, oral sex can lead to a throat infection, though chlamydia in the throat is less common and often clears on its own.
You can also spread chlamydia to your own eyes by touching them after handling infected genital secretions. This hand-to-eye transmission causes a form of conjunctivitis that, if untreated, can lead to lasting eye damage. A pregnant person with chlamydia can pass the infection to their baby during delivery, which may cause eye infections or pneumonia in the newborn.
You Cannot Get It From Toilet Seats or Casual Contact
The bacterium that causes chlamydia cannot survive outside the human body. You won’t catch it from toilet seats, swimming pools, shared towels, doorknobs, or casual contact like hugging or shaking hands. It requires direct contact with infected mucous membranes or secretions. This is not a germ that lingers on surfaces waiting to infect someone.
How the Bacteria Infects Your Body
Once the chlamydia bacterium lands on a mucosal surface, it attaches to the cells lining that tissue through an initial electrostatic attraction. This first contact is brief and reversible, but what follows happens fast. The bacterium injects proteins directly into the host cell using a needle-like structure on its surface. These proteins hijack the cell’s internal scaffolding, causing tiny finger-like projections to rise up around the bacterium and pull it inside.
Once engulfed, the bacterium lives and replicates within a protective bubble inside the cell, hidden from your immune system. This is why chlamydia can persist for months without triggering obvious symptoms. The infection cycle continues as new bacteria burst out and infect neighboring cells.
Most People Have No Symptoms
The biggest reason chlamydia spreads so effectively is that it’s usually invisible. At least 70% of women with a genital chlamydia infection have no symptoms or only mild ones that wouldn’t prompt a doctor visit. Among men, about 50% of infections are completely silent. People can carry and transmit the bacteria for weeks or months without ever realizing anything is wrong.
When symptoms do appear, they typically include unusual discharge, burning during urination, or pain during sex. In men, testicular pain or swelling sometimes develops. In women, bleeding between periods or pelvic pain can signal that the infection has started to spread. But the absence of symptoms is the norm, not the exception, which is why routine screening matters far more than waiting for something to feel off.
When Chlamydia Becomes Detectable
If you’ve had a potential exposure, timing your test correctly is important. There’s limited data on exactly how soon modern DNA-based tests can pick up chlamydia after sex. Clinical guidelines generally recommend waiting at least two weeks, with many experts suggesting three weeks for the most reliable result. Some clinicians advise testing immediately and then repeating the test three to five weeks later to catch infections that hadn’t reached detectable levels the first time around.
Testing typically involves a urine sample or a swab from the infected site (cervix, rectum, or throat). The standard tests used today are highly accurate once the bacteria has had time to establish itself.
Reinfection Is Common
Curing chlamydia once does not protect you from getting it again. Your body does not build lasting immunity to the bacterium after an infection. Research from the California Department of Public Health shows that as many as 20% of women test positive for chlamydia again within six months of their initial treatment. Most reinfections happen in that first six-month window, often because a sexual partner wasn’t treated at the same time.
This is why both partners need treatment when one tests positive. If only one person takes antibiotics and then resumes unprotected sex with the untreated partner, the infection simply bounces back. Retesting about three months after treatment helps catch these reinfections early, before they cause complications or spread further.
Who Is Most at Risk
Anyone who is sexually active can get chlamydia, but certain factors raise your likelihood. Having a new sexual partner, having multiple partners, or not using condoms consistently all increase risk. Younger people, particularly those under 25, have the highest infection rates, which is why annual screening is recommended for all sexually active women in that age group.
Men who have sex with men face elevated risk for rectal and throat infections specifically, and screening at those sites is recommended regardless of whether symptoms are present. Previous chlamydia infection is itself a risk factor, since the circumstances that led to the first infection (partner dynamics, condom use patterns) often haven’t changed by the time treatment is complete.