Chlamydia spreads through sexual contact, specifically vaginal, anal, or oral sex without a condom with someone who has the infection. It cannot spread through casual contact like hugging, sharing drinks, or sitting on toilet seats. What makes chlamydia especially easy to catch is that most people who have it don’t know it: about 75% of infected women and 50% of infected men have no symptoms at all, meaning a partner can pass it along without either person realizing anything is wrong.
Sexual Contact Is the Primary Route
Chlamydia is caused by the bacterium Chlamydia trachomatis, which lives in the mucous membranes of the body, particularly in the vagina, penis, rectum, and throat. It spreads when these membranes come into contact during sex. The estimated transmission probability is about 4.5% per single act of unprotected vaginal intercourse, which sounds low but adds up quickly over repeated encounters with an infected partner.
Vaginal sex is the most commonly discussed route, but anal and oral sex carry real risk too. Receptive anal sex can lead to a rectal chlamydia infection, and the bacteria can also spread from the vagina to the rectum without anal sex occurring. Oral sex can result in a throat infection, though this is less common. These non-genital infections are easy to miss because they rarely cause noticeable symptoms and aren’t detected by a standard urine test.
Sharing Sex Toys
Chlamydia can also pass between partners through shared sex toys. The NHS lists chlamydia among the infections that can spread this way. If you share toys, washing them between uses and covering them with a new condom each time significantly reduces the risk.
Spreading It to Your Own Eyes
If you have a genital chlamydia infection, you can transfer the bacteria to your eyes by touching your genitals and then touching your eyes without washing your hands. This is called auto-inoculation, and it’s a well-documented cause of chlamydial conjunctivitis in adults. The resulting eye infection causes redness, discharge, and irritation, and needs treatment to avoid complications.
Passing It During Childbirth
A pregnant person with an untreated chlamydia infection can pass the bacteria to their baby during vaginal delivery. This can cause eye infections or pneumonia in the newborn. Prenatal screening catches most cases before delivery. Current guidelines recommend testing all pregnant people under 25, and those 25 and older who have risk factors, with retesting in the third trimester for those at continued risk.
Ways You Cannot Get Chlamydia
Chlamydia bacteria cannot survive outside the body’s mucous membranes. They die quickly when exposed to air or dry surfaces. This means you cannot get chlamydia from toilet seats, swimming pools, shared towels, doorknobs, or cutlery. You also can’t catch it from a hug, a handshake, or a cough. The infection requires direct mucous membrane contact with infected fluids.
Why Asymptomatic Infections Matter
The reason chlamydia spreads so effectively is that most infections are silent. Three out of four women and half of men with chlamydia experience no symptoms whatsoever. Without testing, these infections go undiagnosed and untreated, sometimes for months or years. During that entire time, the person can transmit the infection to sexual partners. This is why routine screening, not just testing when symptoms appear, is so important for catching and stopping transmission.
Reinfection After Treatment
Getting treated for chlamydia does not protect you from getting it again. Reinfection is common, and it usually happens because a sexual partner wasn’t treated at the same time. If you’re cured but your partner still carries the bacteria, you’ll likely get reinfected the next time you have unprotected sex. This “ping-pong” pattern is common enough that the CDC recommends retesting about three months after treatment. Both partners need to complete treatment and avoid sex until the course is finished to break the cycle.
Who Should Get Tested
Because chlamydia so often flies under the radar, screening guidelines cast a wide net. Sexually active women under 25 should be tested annually. Women 25 and older should be tested if they have a new partner, more than one partner, a partner who has other partners, inconsistent condom use outside a mutually monogamous relationship, or a history of STIs. Men who have sex with men should be tested at least annually at all sites of contact, and every three to six months if they’re on PrEP, have HIV, or have multiple partners.
For anyone with HIV, screening is recommended at the first evaluation and at least once a year after that. Transgender and gender diverse individuals should follow screening recommendations based on their anatomy and sexual practices. Testing is simple, usually a urine sample or a swab, and treatment is a short course of antibiotics.