Chlamydia spreads through sexual contact with an infected person. It can be transmitted during vaginal, anal, or oral sex, even when the infected person has no symptoms. In fact, 75% of women and 50% of men with chlamydia never develop noticeable symptoms, which is why the infection spreads so easily and often without either partner realizing it.
Vaginal and Anal Sex
The most common way chlamydia spreads is through unprotected vaginal or anal intercourse. The bacteria infect the mucous membranes of the genitals and rectum, essentially hijacking cells in those tissues to replicate. Once the bacteria attach to a cell, they get pulled inside and create a protected compartment where they grow and multiply without being destroyed by your immune system’s normal defenses.
You don’t need to have rough sex or visible sores for transmission to occur. Simple contact between infected mucous membranes is enough. Anal sex carries a particular risk because the lining of the rectum is thin and easily irritated, and rectal chlamydia infections often produce no symptoms at all, making them easy to miss and pass along.
Oral Sex
Chlamydia can infect the throat, though this route is less commonly discussed. If you perform oral sex on someone who has a genital or rectal chlamydia infection, you can develop a throat infection. The reverse is also possible: receiving oral sex from someone with chlamydia in their throat can lead to a genital infection.
Throat chlamydia infections tend to cause fewer problems than genital or rectal ones, and many people never realize they have one. But a throat infection still makes you contagious. Having chlamydia in the throat may actually make it easier to spread the bacteria to partners through oral sex, according to the CDC.
During Childbirth
A pregnant person with an untreated chlamydia infection can pass the bacteria to their baby during vaginal delivery. The vertical transmission rate to the newborn’s eyes is roughly 30%, which can cause a serious eye infection called neonatal conjunctivitis. The bacteria can also infect the infant’s lungs, potentially leading to pneumonia in the first few months of life.
How You Cannot Get Chlamydia
Chlamydia bacteria cannot survive outside the human body for more than a very short time. You cannot get chlamydia from a toilet seat, a swimming pool, shared towels, or casual contact like hugging or shaking hands. The bacteria need warm, moist mucous membranes to survive, and hard surfaces like toilet seats don’t provide that environment. There is virtually zero chance of contracting any STI from a toilet seat under normal circumstances.
You also can’t get chlamydia from kissing, sharing food or drinks, or breathing the same air as an infected person. It requires direct sexual contact with infected tissue.
Why It Spreads So Easily
The biggest reason chlamydia is so widespread is the asymptomatic rate. When three out of four infected women and half of infected men feel completely fine, they have no reason to suspect they’re carrying and transmitting the bacteria. Someone can be infected for weeks, months, or even longer without knowing it, having sex with new partners the entire time.
When symptoms do appear, they typically show up one to three weeks after exposure. But the absence of symptoms doesn’t mean the infection is harmless. Untreated chlamydia can cause pelvic inflammatory disease in women, which can lead to chronic pain and fertility problems. In men, it can cause painful swelling in the reproductive tract.
Reinfection After Treatment
Having chlamydia once does not protect you from getting it again. Reinfection is extremely common, and the CDC notes that most repeat cases aren’t treatment failures. They happen because a person’s sexual partner wasn’t treated at the same time, or because the person started having sex with a new infected partner. This is why the CDC recommends retesting about three months after treatment, regardless of whether you believe your partner was also treated.
How Condoms Reduce the Risk
Consistent condom use reduces the risk of chlamydia transmission by an estimated 50% to 90%. That’s a wide range because real-world effectiveness depends on whether condoms are used correctly every time and throughout the entire sexual encounter, not just at the end. Condoms work by creating a physical barrier between mucous membranes, blocking the bacteria from reaching new cells.
For oral sex, dental dams provide a similar barrier. No method short of abstinence eliminates the risk entirely, but regular screening is a powerful tool because chlamydia is easily curable with antibiotics when caught. Testing is reliable as soon as one week after exposure and catches nearly all infections by two weeks.