Chlamydia spreads primarily through vaginal, anal, or oral sex with someone who is infected. It is the most commonly reported bacterial sexually transmitted infection in the United States, with over 1.5 million cases reported in 2024 alone. What makes it especially easy to contract is that most people who carry it have no symptoms at all, meaning you can get it from a partner who genuinely doesn’t know they’re infected.
Sexual Contact Is the Primary Route
The bacterium that causes chlamydia, Chlamydia trachomatis, infects the moist mucous membranes that line the genitals, rectum, and throat. Any 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 have penetrative sex to contract it; contact between genital mucous membranes is enough.
Ejaculation is not required for transmission. The bacteria live in the tissue itself, not exclusively in semen or vaginal fluid, so any direct contact with infected tissue poses a risk. Receptive anal sex carries a particularly high transmission risk because the rectal lining is thin and vulnerable to the bacterium. Oral sex is a less efficient route, but it can still lead to a chlamydial throat infection that often produces no symptoms.
Why So Many People Spread It Without Knowing
At least 70% of genital chlamydia infections in women and 50% in men produce no noticeable symptoms at the time of diagnosis. This is the single biggest reason chlamydia spreads so effectively. A person can carry the infection for weeks, months, or even longer, passing it to sexual partners during that entire window. When symptoms do appear, they typically show up several weeks after exposure, not days, which further delays diagnosis and treatment.
For women, symptoms (when present) include unusual vaginal discharge, burning during urination, or bleeding between periods. Men may notice discharge from the penis, burning with urination, or pain in the testicles. Rectal infections can cause discharge, pain, or bleeding, but again, many rectal infections are completely silent. The absence of symptoms does not mean the infection is mild or harmless. Untreated chlamydia can cause serious reproductive damage over time, particularly in women, where it can lead to pelvic inflammatory disease and fertility problems.
How the Bacterium Infects Your Cells
Chlamydia trachomatis is an unusual bacterium because it can only survive inside human cells. It cannot grow on surfaces, in water, or in the open air. Once it reaches a suitable mucous membrane, it attaches to the surface of epithelial cells (the cells lining your genital tract, rectum, or throat) through an electrostatic attraction, almost like a weak magnet. This initial attachment is followed by a stronger, irreversible binding step.
From there, the bacterium essentially tricks your cells into swallowing it. It injects specialized proteins through the cell membrane that reorganize the cell’s internal scaffolding, creating tiny finger-like projections that wrap around the bacterium and pull it inside. Once safely enclosed in a protective bubble within the cell, the bacterium shifts into its reproductive form, multiplying until the cell bursts and releases new bacteria to infect neighboring cells. This entire cycle is why chlamydia requires living human tissue and cannot survive on inanimate objects.
Transmission During Childbirth
A pregnant person with an untreated chlamydia infection can pass the bacterium to their baby during vaginal delivery. As the infant moves through the birth canal, the bacteria can infect the baby’s eyes, causing a condition called neonatal conjunctivitis, or reach the lungs, potentially causing pneumonia in the first few months of life. This is one reason routine chlamydia screening is standard during prenatal care.
Eye Infections From Hand Contact
Chlamydia can infect the eyes, and this happens more often than people realize. The most common way is auto-inoculation: touching your own genitals (or a partner’s) and then touching your eyes without washing your hands. The bacterium transfers from genital tissue to the thin mucous membrane covering the eye, where it can establish an infection. Chlamydial conjunctivitis causes redness, discharge, and swelling, and it requires treatment to avoid long-term damage to the eye.
This same hand-to-eye route means that even without traditional intercourse, the bacteria can reach new tissue sites on your own body or a partner’s body through touch.
What Doesn’t Transmit Chlamydia
Because chlamydia requires specific living human tissue to survive, it cannot be transmitted through casual, non-sexual contact. You will not get chlamydia from:
- Toilet seats or bathroom surfaces. The bacterium dies quickly once it leaves the body and dries out. Even if infected fluid were present, it would need to contact your cervix, urethra, or another mucous membrane, not just your skin.
- Swimming pools or hot tubs. The dilution in water and the chlorine used in most pools and spas make transmission effectively impossible.
- Sharing towels, bed linens, or clothing. While theoretically a very fresh, wet droplet of infected fluid could carry live bacteria briefly, the organism cannot survive long enough on fabric for this to be a realistic transmission route.
- Hugging, shaking hands, or sharing food. Skin-to-skin contact that doesn’t involve mucous membranes poses no risk.
Who Is Most at Risk
Anyone who is sexually active can contract chlamydia, but certain factors raise the likelihood. Having multiple sexual partners, having a new partner, or not using condoms consistently all increase your exposure. Younger people, particularly those under 25, have the highest rates of infection, partly because the cervical tissue in younger women is more biologically susceptible and partly because of behavioral patterns like less consistent condom use.
Having had chlamydia before does not protect you from getting it again. Reinfection is common, especially if a sexual partner was not treated at the same time. If you’re treated for chlamydia, any recent partners need treatment too, or the infection will simply pass back and forth between you.
How to Reduce Your Risk
Condoms, when used correctly every time, significantly reduce the risk of chlamydia transmission, though they don’t eliminate it entirely since the bacteria can be present on tissue not covered by the condom. Regular screening is the most effective tool for catching the infection early. Annual screening is recommended for sexually active women under 25 and for anyone with new or multiple partners. Because the infection is so often silent, testing is the only reliable way to know your status.
Chlamydia is easily cured with a short course of antibiotics. The challenge is not treatment but detection. Given that over a million and a half cases were reported in the U.S. in 2024, and many more likely went undiagnosed, routine testing remains the most practical defense against both contracting the infection and unknowingly passing it to others.