Chlamydia spreads through sexual contact, specifically vaginal, anal, or oral sex without a condom with someone who has the infection. It can also pass from a pregnant person to their baby during childbirth. You cannot get chlamydia from toilet seats, swimming pools, or casual contact like hugging or sharing food.
How the Infection Enters Your Body
Chlamydia is caused by the bacterium Chlamydia trachomatis, which targets the thin, moist tissue lining your genital tract, rectum, and throat. When the bacteria reach these surfaces during sex, they attach to cells in the epithelium (the innermost layer of tissue) and essentially trick those cells into absorbing them. Within about two hours, the bacteria begin transforming into a form that can rapidly multiply. This all happens without any obvious signs, which is why most people with chlamydia have no idea they’re infected.
The infection typically starts in the lower genital tract: the cervix in women or the urethra in men. If left untreated, it can spread upward to the uterus and fallopian tubes in women, potentially causing pelvic inflammatory disease and long-term fertility problems. In men, it can reach the tube that carries sperm from the testicles, though this is less common.
Specific Ways Chlamydia Spreads
The three main sexual routes are vaginal, anal, and oral sex. Any contact between mucous membranes and infected genital fluids creates an opportunity for transmission. You don’t need to experience ejaculation for the bacteria to pass between partners.
- Vaginal sex is the most common route. The bacteria transfer between the penis and the vaginal lining in either direction.
- Anal sex can cause a rectal infection, particularly for the receptive partner. Rectal chlamydia can also develop when the bacteria spread from an existing vaginal infection without anal sex occurring.
- Oral sex can transmit chlamydia to the throat, though throat infections are less common and often clear on their own.
- Childbirth is the one non-sexual transmission route. A baby passing through an infected birth canal can develop eye infections or pneumonia.
Sharing sex toys can also transfer the bacteria if a toy is used by one partner and then the other without cleaning or covering it with a new condom.
What Doesn’t Cause Chlamydia
Chlamydia bacteria cannot survive outside the human body for more than a very short time. They need warm, moist mucous membranes to stay alive, so hard surfaces like toilet seats pose virtually zero risk. You also can’t contract chlamydia from swimming pools, hot tubs, doorknobs, or shared towels. Kissing, hugging, and sharing drinks or utensils are not transmission routes. The bacterium is fragile outside its preferred environment, which is why it requires direct mucous membrane contact to spread.
Who Is Most Likely to Get Infected
Chlamydia is the most commonly reported bacterial sexually transmitted infection in the United States, with more than 2.2 million combined cases of chlamydia, gonorrhea, and syphilis reported in 2024. Age is the single strongest predictor of infection. Prevalence is highest among sexually active people under 25, with rates dropping steadily in older age groups. Among all people aged 14 to 39, the overall prevalence is about 1.7%, but for sexually active women aged 14 to 24, that number jumps to 4.7%.
Having more sexual partners increases your risk. People who reported two or more partners in the past year had a chlamydia prevalence of 3.2%, compared to 1.4% among those with one partner. This makes sense given more partners means more potential exposures, especially since most carriers show no symptoms and don’t know they’re contagious.
Significant racial disparities exist in chlamydia rates, driven largely by differences in access to screening, treatment, and healthcare. These disparities reflect systemic inequities rather than biological differences.
Why Asymptomatic Infection Matters
The majority of chlamydia infections produce no symptoms at all, which is the main reason the infection spreads so easily. When symptoms do appear, they typically show up one to three weeks after exposure, though some people don’t notice anything for months. Common signs include unusual discharge, burning during urination, and in women, bleeding between periods or pain during sex. In men, testicular pain or swelling can occur but is uncommon.
Because silent infections are the norm, regular screening is the most reliable way to catch chlamydia early. The CDC recommends annual screening for all sexually active women under 25 and for older women with new or multiple partners.
Reinfection Is Common
Having chlamydia once does not protect you from getting it again. Your body does not build lasting immunity the way it does with some viral infections. Research shows that about 20% of women who test positive for chlamydia become reinfected within six months of treatment. Most reinfections happen in that first six-month window, usually because a sexual partner wasn’t treated at the same time or because a new exposure occurred shortly after completing antibiotics.
This is why retesting three months after treatment is standard practice. If your partner wasn’t treated, the bacteria will simply pass back to you the next time you have sex, even if your own course of antibiotics was completed perfectly. Both partners need treatment at the same time for it to work.
How to Reduce Your Risk
Condoms are the most effective barrier against chlamydia during vaginal and anal sex. They don’t eliminate risk entirely, since skin-to-skin contact around the genitals can still occur, but they dramatically reduce it. For oral sex, dental dams provide a similar barrier. Limiting your number of sexual partners and choosing partners who have been recently tested also lowers your odds. If you’re in a new sexual relationship, getting tested together before having unprotected sex is the most straightforward way to know where you both stand.