Chlamydia spreads through sexual contact with an infected person. It’s caused by the bacterium Chlamydia trachomatis, and it passes from one person to another during vaginal, anal, or oral sex without a condom. Roughly 75% of women and 50% of men with chlamydia have no symptoms at all, which means most people who transmit it don’t know they’re infected.
How the Bacteria Spread During Sex
Chlamydia trachomatis lives in the mucous membranes of the body, particularly the genitals, rectum, and throat. During unprotected sex, the bacteria transfer through direct contact with these infected tissues or the fluids they produce. You don’t need to have a particularly long or intense encounter. Brief contact with infected mucous membranes is enough.
Vaginal sex is the most common route. Anal sex, both giving and receiving, can spread the infection to or from the rectum. Oral sex can transmit it to the throat, though this is less common. The bacteria can also spread from one site on your own body to another. For example, a vaginal infection can migrate to the rectum without anal sex ever occurring.
A pregnant person with chlamydia can also pass the infection to their baby during vaginal delivery. This can cause eye infections or pneumonia in the newborn, which is why screening during pregnancy is standard practice.
What You Can’t Get It From
Chlamydia trachomatis cannot survive outside the human body for more than a very short time. That means you won’t catch it from toilet seats, swimming pools, shared towels, doorknobs, or casual skin contact like hugging or handshakes. The bacterium needs warm, moist human tissue to survive and replicate. It’s essentially a parasite of human cells, hijacking your body’s own resources to grow and multiply.
You also can’t get chlamydia from kissing, sharing food, or breathing the same air as someone who’s infected. It requires direct mucous membrane contact during sexual activity.
Why So Many People Don’t Know They Have It
The biggest reason chlamydia spreads so effectively is that most infections are silent. Three out of four women and half of men experience no symptoms whatsoever. When symptoms do appear, they typically show up 7 to 21 days after exposure, though in some cases it can take months. Common signs include unusual discharge, burning during urination, or pain during sex, but the absence of these signs tells you nothing about whether you’re infected.
This silent nature creates a cycle. Someone picks up the infection without knowing it, continues having sex, and passes it to new partners who also may not realize they’re carrying it. This is why routine screening matters far more for chlamydia than waiting for symptoms to prompt a visit.
Who’s Most Likely to Get It
Chlamydia is the most common bacterial sexually transmitted infection worldwide. In the United States alone, it accounts for the bulk of the more than 2.2 million combined chlamydia, gonorrhea, and syphilis cases reported in 2024. Young, sexually active people are at highest risk, particularly those under 25. Having multiple partners or a new partner increases your chances, as does inconsistent condom use.
Having had chlamydia before doesn’t protect you from getting it again. Your body does not build lasting immunity. Reinfection rates are strikingly high: studies have found that anywhere from 12% to 34% of treated women test positive again within a year. Some of these are true reinfections from a partner who wasn’t treated. Others result from the bacteria persisting in the rectum and reinfecting other sites. This is why both you and any recent sexual partners need treatment at the same time.
How It Gets Inside Your Cells
What makes chlamydia unusual among bacteria is that it can only reproduce inside your cells. The bacterium exists in two forms. Outside a cell, it takes on a small, tough, infectious form that can latch onto the surface of cells lining your genitals, rectum, or throat. Once it attaches, it essentially injects proteins through a needle-like structure that tricks the cell into pulling it inside.
Once safely inside, the bacterium switches into a larger form optimized for replication. It feeds off your cell’s own nutrients, copies itself repeatedly, then converts back into the infectious form and bursts out of the cell or pinches off in a bubble of cell membrane. Those newly released bacteria then infect neighboring cells, and the cycle continues. This intracellular lifestyle is part of why your immune system has a hard time clearing the infection on its own and why reinfection is so common.
When and How to Get Tested
If you think you’ve been exposed, the standard test uses a urine sample or a swab from the vagina, rectum, or throat. The test detects the bacterium’s genetic material and is highly accurate, but it needs enough bacteria present to pick up a signal. Most infections become detectable about one week after exposure, and waiting two weeks catches nearly all cases.
Testing too early, within the first few days, risks a false negative. If your result comes back negative but you were tested early, repeating the test after the two-week mark gives a more reliable answer. Because so many infections produce no symptoms, sexually active people under 25 and anyone with a new partner benefit from regular screening even when nothing feels wrong.