Chlamydia spreads primarily through vaginal, anal, or oral sex with someone who has the infection. It can also pass from a pregnant person to their baby during delivery, and in rare cases, from infected genital fluids to the eyes. You do not need to have symptoms to transmit it, and most people who carry chlamydia don’t know they have it.
Vaginal and Anal Sex
Unprotected vaginal and anal intercourse are the most common ways chlamydia spreads. The bacterium infects mucous membranes, the moist tissue lining the genitals, rectum, and throat. During sex, the bacteria transfer through direct contact with these tissues and the fluids they produce. Ejaculation does not have to occur for transmission to happen.
The estimated risk of catching chlamydia from a single unprotected sexual encounter is roughly 4.5%, based on modeling published in the journal Sexually Transmitted Infections. That may sound low for a single act, but risk accumulates quickly over repeated encounters with an infected partner, and the infection is extremely common, especially among people under 25.
Oral Sex
Chlamydia can infect the throat when someone performs oral sex on a partner who has a genital or rectal infection. It can also move in the other direction: a throat infection can spread to a partner’s genitals during oral sex. Throat infections tend to cause few or no symptoms, which means people can unknowingly pass the bacteria along. A chlamydia infection in the throat may be less physically harmful than one in the genitals or rectum, but it can still increase susceptibility to other infections, including HIV.
Shared Sex Toys
Sharing sex toys is a recognized route of transmission. The NHS lists chlamydia among the infections that can pass between partners through shared toys. If a toy comes into contact with infected fluids and is then used by another person without being cleaned, the bacteria can transfer to the new user’s mucous membranes. Washing toys thoroughly between users and placing a fresh condom on them each time significantly reduces this risk.
Hand-to-Eye Contact
Chlamydia can also infect the eyes, a condition known as chlamydial conjunctivitis. This typically happens when infected genital secretions are transferred to the eye by hand. Symptoms include red, irritated eyes, swollen lids, discharge, lids sticking together, sensitivity to light, and sometimes blurred vision. The infection can affect one or both eyes. Simple handwashing after any genital contact is the most effective way to prevent this.
During Childbirth
A pregnant person with an untreated chlamydia infection can pass the bacteria to their baby during vaginal delivery. As the baby moves through the birth canal, it comes into direct contact with infected tissue. This can cause serious health problems for the newborn, including eye infections and pneumonia. Routine screening during pregnancy catches most cases so they can be treated before delivery.
Ways You Cannot Catch It
Chlamydia bacteria cannot survive long outside the human body. There is virtually zero chance of picking up the infection from a toilet seat, swimming pool, hot tub, shared towel, or doorknob. The bacteria need the warm, moist environment of mucous membranes to survive. Casual contact like hugging, kissing on the cheek, sharing food or drinks, or sitting next to someone does not transmit chlamydia.
Why Most People Don’t Realize They’re Contagious
One of the biggest reasons chlamydia spreads so easily is that most carriers have no symptoms at all. At least 70% of women and 50% of men with a genital chlamydia infection show no signs at the time of diagnosis. That means a person can feel perfectly healthy and still transmit the bacteria to every sexual partner. This is why routine screening matters so much: the only reliable way to know your status is to get tested.
When symptoms do appear, they typically show up one to three weeks after exposure. In women, this can include unusual vaginal discharge, burning during urination, or bleeding between periods. In men, symptoms often involve discharge from the penis, burning with urination, or pain and swelling in one or both testicles. Rectal infections may cause pain, discharge, or bleeding regardless of sex. But waiting for symptoms is not a reliable strategy, since most infections stay silent.
How to Lower Your Risk
Condoms and dental dams, when used consistently and correctly, significantly reduce the chance of transmission during vaginal, anal, and oral sex. They don’t eliminate risk entirely because chlamydia spreads through any mucosal contact, but they create a physical barrier over the primary infection sites. Getting tested regularly, particularly if you have new or multiple partners, is the single most effective way to break the chain of transmission. Chlamydia is curable with a short course of antibiotics, so catching it early protects both you and your partners.