Chlamydia spreads through sexual contact with an infected person, primarily during vaginal, anal, or oral sex without a condom. It can also pass from mother to baby during childbirth. What makes chlamydia especially easy to transmit is that most people who carry it have no idea they’re infected.
Sexual Contact Is the Primary Route
Chlamydia is caused by the bacterium Chlamydia trachomatis, which lives in infected genital fluids, including semen, pre-ejaculate, and vaginal secretions. The infection passes when these fluids come into contact with the mucous membranes of a partner’s genitals, rectum, or throat. You don’t need to experience ejaculation for transmission to occur. Any unprotected sexual contact that involves these areas creates an opportunity for the bacteria to spread.
Vaginal sex is the most common route of transmission. Anal sex carries risk as well, both for the receptive and insertive partner, though receptive anal sex poses the higher risk. The infection can also spread from one body site to another in the same person. For example, someone with a vaginal infection can develop a rectal infection without having had anal sex.
Oral sex is a less efficient route, but it can still transmit chlamydia to the throat. Throat infections often cause no symptoms at all, which means they can go unnoticed for long periods.
Research published in the journal Sexually Transmitted Infections estimated the per-act transmission probability at roughly 4.5% for a single unprotected vaginal encounter. That may sound low, but the risk compounds quickly with repeated exposure, and the number likely varies depending on factors like bacterial load and whether other infections are present.
Most Carriers Have No Symptoms
The single biggest factor driving chlamydia transmission is that the majority of infected people feel perfectly fine. About 75% of women and 50% of men with chlamydia experience no symptoms whatsoever. That means a person can carry and spread the infection for weeks or months without knowing it, passing it to new partners along the way.
When symptoms do appear, they typically show up one to three weeks after exposure, though they can take longer. In women, symptoms may include unusual vaginal discharge, burning during urination, or bleeding between periods. In men, the most common signs are discharge from the penis and a burning sensation when urinating. Rectal infections can cause discharge, pain, or bleeding, but often produce no symptoms at all.
Because the infection is so frequently silent, routine screening is the only reliable way to catch it early. The CDC recommends annual chlamydia screening for all sexually active women under 25, and for older women with risk factors like a new partner, multiple partners, or inconsistent condom use. Men who have sex with men should be screened at least annually at all sites of contact, and every three to six months if they’re at higher risk. Screening recommendations for transgender and gender diverse individuals are based on anatomy and sexual behavior rather than gender identity alone.
Transmission During Childbirth
A pregnant person with an untreated chlamydia infection can pass the bacteria to their baby during vaginal delivery. The newborn’s eyes and respiratory tract are the most vulnerable. Chlamydia is a leading cause of conjunctivitis (eye infection) in newborns and can also cause early infant pneumonia. This is why chlamydia screening is recommended for all pregnant individuals under 25, with retesting in the third trimester for those at continued risk.
Eye Infections From Hand Contact
Chlamydia can infect the eyes through a less obvious route: touching your eyes after handling infected genital secretions. This hand-to-eye spread causes chlamydial conjunctivitis, which leads to redness, discharge, and swelling that can persist for weeks if untreated. It’s uncommon compared to genital transmission, but it’s a real pathway, and a good reason to wash your hands after sexual contact.
What Doesn’t Spread Chlamydia
Chlamydia bacteria cannot survive long outside the human body. You cannot get chlamydia from a toilet seat, a swimming pool, a hot tub, or by sharing food, drinks, or utensils. Hugging, kissing, and casual skin-to-skin contact don’t transmit it either. The bacteria require the warm, moist environment of mucous membranes to survive, which is why direct sexual contact is essentially the only way it spreads between adults.
There’s also no meaningful risk from shared towels or bedding. While certain parasites like pubic lice can theoretically survive briefly on fabrics, the bacteria responsible for chlamydia die rapidly on surfaces.
How Condoms and Screening Reduce Risk
Consistent, correct condom use during vaginal, anal, and oral sex significantly reduces the risk of chlamydia transmission. Condoms create a physical barrier that prevents infected fluids from reaching a partner’s mucous membranes. They aren’t 100% effective, partly because they may not cover all potentially infected areas, but they remain one of the most practical tools for prevention.
Because so many infections are asymptomatic, regular screening matters just as much as barrier protection. Getting tested allows you to catch an infection before you unknowingly pass it to someone else. If you test positive, your recent sexual partners need to be notified and tested too, since reinfection from an untreated partner is common. The CDC recommends retesting about three months after treatment to catch any reinfection early.
Chlamydia is curable with antibiotics, and treatment is straightforward. But left untreated, it can cause serious complications, including pelvic inflammatory disease in women, which can lead to chronic pain and fertility problems. Early detection through screening is the most reliable way to prevent both transmission and long-term damage.