Chlamydia is caused by a bacterium called Chlamydia trachomatis, spread almost exclusively through sexual contact. It is the most commonly reported bacterial sexually transmitted infection in the United States, with roughly 1.5 million cases recorded in 2024 alone.
The Bacterium Behind the Infection
Chlamydia trachomatis is an unusual type of bacterium. Unlike most bacteria, which can survive and multiply on their own, this one can only reproduce inside human cells. It hijacks your cells’ resources to copy itself, which is why it’s classified as an “obligate intracellular” pathogen. This trait also makes it harder for your immune system to detect and clear, which is a big reason why so many infections go unnoticed.
The bacterium exists in two forms. The first is a tough, compact particle that can survive outside cells long enough to pass between people during sexual contact. Once this particle enters your body and attaches to a cell (typically in the lining of the genitals, rectum, or throat), it slips inside and transforms into a larger, actively dividing form. Over the next 36 to 50 hours, it multiplies rapidly within a protective pocket inside the cell. Eventually the host cell ruptures, releasing hundreds of new infectious particles that can infect neighboring cells or be transmitted to a sexual partner.
How Chlamydia Spreads
The bacteria travel in vaginal fluid and semen. You can get chlamydia through vaginal, anal, or oral sex without a condom with someone who has the infection. Penetration and ejaculation aren’t strictly necessary: any genital-to-genital or genital-to-mucous-membrane contact that transfers these fluids can do it. Sharing sex toys with an infected person is another, less common route.
The infection can also spread from one site to another on the same person. Someone with a genital infection can transfer the bacteria to their own eyes by touching infected fluid and then their face, causing a form of conjunctivitis. Rectal infections can develop either from receptive anal sex or from bacteria spreading from an infected vagina.
A pregnant person with chlamydia can pass the infection to their baby during vaginal delivery. This can cause eye infections or pneumonia in the newborn.
Chlamydia does not spread through casual contact like hugging, sharing food, or sitting on toilet seats. The bacterium is fragile outside the body and needs direct mucous membrane contact to establish an infection.
Why Most People Don’t Know They Have It
About 75% of women and 50% of men with chlamydia have no symptoms at all. This is the central problem with the infection: people carry and spread it without realizing anything is wrong. When symptoms do appear, they typically show up one to three weeks after exposure and can include unusual genital discharge, burning during urination, or pain during sex. In men, testicular pain or swelling sometimes occurs. In women, bleeding between periods or after sex can be a sign.
Because the infection is so often silent, routine screening is the primary way it gets caught. The standard test uses a technology called nucleic acid amplification, which detects the bacterium’s genetic material with over 90% sensitivity and 99% or higher specificity. It picks up 20% to 50% more infections than older testing methods. A simple urine sample or swab is all that’s needed.
Who Is Most at Risk
Age is the single strongest predictor of chlamydia infection. Among pregnant women studied in one large analysis, 14.6% of those under 25 tested positive, compared to 4.3% of those aged 25 to 29 and just 1.7% of those 30 and older. Women under 25 had seven times the odds of infection compared to women over 30.
Why younger people are hit hardest isn’t entirely settled. Part of it is behavioral: younger adults tend to have more new sexual partners and are less likely to use condoms consistently. Part of it may be biological. In younger women, the type of tissue that lines the cervix extends further outward in a pattern called cervical ectropion, which may make those cells more accessible to the bacteria. There’s also a possible immunological component, since older adults who’ve been exposed to chlamydia before may carry some degree of acquired resistance.
Other factors that increase risk include having multiple sexual partners, having a partner who has an STI, not using barrier protection, and having had chlamydia before (reinfection is common).
What Happens If It Goes Untreated
Left alone, chlamydia doesn’t just sit quietly. In women, the bacteria can climb from the cervix into the uterus and fallopian tubes, triggering pelvic inflammatory disease. The immune system’s response to the infection, particularly repeated or prolonged infections, causes inflammation and scarring inside the fallopian tubes. The bacteria provoke the release of proteins that break down tissue and trigger fibrosis, gradually narrowing or blocking the tubes entirely. Infected cells in the fallopian tubes lose the tiny hair-like structures that help move eggs, and the damage spreads beyond just the directly infected cells through inflammatory signaling. This scarring can lead to chronic pelvic pain, ectopic pregnancy, and infertility.
In men, untreated chlamydia can cause infection of the tube that carries sperm from the testicle, leading to pain and, rarely, fertility problems. In both sexes, untreated infection can also increase susceptibility to other STIs, including HIV.
The good news is that chlamydia is easily curable with a short course of antibiotics when caught early. Because reinfection is common, retesting about three months after treatment is recommended to make sure the infection hasn’t returned, often through an untreated partner.