Chlamydia spreads through sexual contact, specifically vaginal, anal, or oral sex without a condom with someone who has the infection. It is the most commonly reported bacterial sexually transmitted infection in the United States, with over 1.5 million cases recorded in 2024 alone. Because the majority of infected people never develop symptoms, many pass it along without realizing they have it.
Sexual Contact Is the Primary Route
The bacteria that causes chlamydia, called Chlamydia trachomatis, infects the moist tissue that lines the genitals, rectum, and throat. Any unprotected sexual contact that brings these tissues together can transmit the infection. That includes vaginal sex, anal sex, and oral sex. You don’t need to experience ejaculation for transmission to occur. Direct contact between mucous membranes is enough.
Rectal infections can happen in two ways: through receptive anal sex, or by the bacteria spreading from an existing infection at another site, such as the vagina. This means someone with a genital chlamydia infection can develop a rectal infection without ever having had anal sex. Throat infections from oral sex are less common but do occur.
Why So Many People Spread It Unknowingly
Roughly 75% of women and 50% of men with chlamydia have no symptoms at all. This is the single biggest reason the infection spreads so widely. Someone can carry the bacteria for weeks or months, have sex with multiple partners, and never feel anything unusual. When symptoms do appear, they typically show up one to three weeks after exposure, but this timeline varies and some people remain symptom-free indefinitely.
For women, symptoms can include unusual vaginal discharge, burning during urination, or bleeding between periods. Men may notice discharge from the penis, burning when urinating, or pain in the testicles. Rectal infections can cause discharge, pain, or bleeding regardless of sex. But again, most people experience none of this, which is why routine screening is the only reliable way to catch it.
How the Bacteria Actually Infects You
Chlamydia trachomatis exists in two forms. The first is a small, tough particle designed to survive briefly outside a cell and latch onto a new host. Once this particle contacts the cells lining your genital tract, throat, or rectum, it attaches to the cell membrane and gets pulled inside. Within about two hours, it transforms into a second form that can rapidly multiply. Over the next several hours, the bacteria divides again and again inside the host cell, eventually bursting out to infect neighboring cells and continue the cycle.
This is why chlamydia targets specific body parts. It needs the warm, moist lining of mucous membranes to establish an infection. Dry skin on your hands, arms, or legs isn’t a hospitable environment for it.
Can You Get It From a Toilet Seat or Swimming Pool?
This is one of the most common questions about chlamydia, and the answer is effectively no. While the bacteria can technically survive on surfaces for two to three hours under humid conditions, this doesn’t translate into a realistic transmission risk. The bacteria needs direct contact with mucous membrane tissue to infect you, and the amount of bacteria left on a dry surface like a toilet seat is far too small to cause an infection. You cannot get chlamydia from swimming pools, hot tubs, shared towels, or casual contact like hugging or shaking hands.
Spread From Genitals to Eyes
One lesser-known route is touching your eyes after contact with infected genital secretions. This is called autoinoculation, and it can cause a chlamydial eye infection known as inclusion conjunctivitis. Symptoms include redness, swelling of the eyelids, a mucus-like discharge, and a gritty foreign body sensation. Left untreated, it can cause scarring and affect vision. A sexual partner can also transfer the bacteria to your eyes during intimate contact. Simple handwashing after sexual activity significantly reduces this risk.
Transmission During Childbirth
A pregnant person with an active chlamydia infection can pass the bacteria to their baby during vaginal delivery. As the baby moves through the birth canal, it comes into contact with the infected tissue. This can cause eye infections (neonatal conjunctivitis) or pneumonia in the newborn. This is one of the reasons chlamydia screening is a routine part of prenatal care, since treating the infection before delivery prevents transmission to the baby.
Who Is Most at Risk
Chlamydia affects people of all ages who are sexually active, but rates are highest among young adults. In 2024, women were diagnosed at a higher rate than men (about 550 cases per 100,000 women compared to 335 per 100,000 men), though this gap likely reflects the fact that women are screened more regularly during routine gynecological visits rather than a true difference in infection rates.
Your risk increases with multiple sexual partners, inconsistent condom use, and a history of previous STIs. Having been treated for chlamydia before does not make you immune. Reinfection is common and happens at the same rate as a first infection. Each new unprotected sexual encounter with an infected partner carries the same risk.
How Testing and Treatment Work
Chlamydia is diagnosed with a simple urine test or a swab of the infected area. Testing is accurate about two weeks after potential exposure. If you test too early, the bacteria may not have multiplied enough to be detected.
Treatment is straightforward: a short course of antibiotics clears the infection in the vast majority of cases. You should avoid sexual contact for seven days after completing treatment to prevent passing it to a partner. Retesting about three months after treatment is recommended because reinfection rates are high, often from an untreated partner. If left untreated, chlamydia can lead to serious complications including pelvic inflammatory disease in women (which can cause chronic pain and infertility) and infection of the reproductive tract in men.