Chlamydia is caused by a bacterium called Chlamydia trachomatis, spread through sexual contact. It is the most commonly reported bacterial sexually transmitted infection in the United States, with chlamydia contributing to more than 2.2 million combined cases of chlamydia, gonorrhea, and syphilis reported in 2024. What makes this particular bacterium unusual is that it cannot survive on its own. It is an obligate intracellular parasite, meaning it must get inside your cells to live and reproduce.
The Bacterium Behind the Infection
Chlamydia trachomatis is not like most bacteria. Organisms such as Salmonella or the bacterium that causes tuberculosis can survive both inside and outside human cells. Chlamydia cannot. It depends entirely on a host cell for energy and replication, which is why it went unidentified for so long and why it cannot be grown on a standard lab dish the way other bacteria can.
The bacterium has a particular preference for columnar epithelial cells, the type of tissue that lines the cervix, urethra, rectum, and throat. In women, the endocervix (the inner part of the cervical canal) is the most commonly infected site and serves as the primary reservoir for the infection. From there, the bacteria can ascend into the uterus and fallopian tubes if left untreated.
How the Bacterium Infects Your Cells
Chlamydia has a two-stage life cycle that sets it apart from other infections. It alternates between two forms: a small, tough infectious particle called an elementary body (about 0.3 micrometers across) and a larger, actively dividing form called a reticulate body (about 1 micrometer). Think of the elementary body as a seed. It is metabolically inert, almost spore-like, built to survive outside cells just long enough to reach a new host.
Once an elementary body attaches to a susceptible cell, it gets pulled inside into a membrane-bound compartment called an inclusion. Safely shielded from your immune system, it transforms into the larger reticulate body and begins dividing rapidly. After enough copies have been made, the reticulate bodies convert back into elementary bodies. The host cell eventually bursts open, releasing a new wave of infectious particles that spread to neighboring cells. This entire cycle takes roughly 48 to 72 hours.
How Chlamydia Spreads
Chlamydia spreads through vaginal, anal, or oral sex without a condom with someone who is infected. The bacteria pass from one person to another through direct contact with infected mucosal tissue or secretions. You do not need to have penetrative sex to transmit it; any contact with an infected site can be enough.
The infection can also establish itself in the rectum, either through receptive anal sex or by spreading from another infected area such as the vagina. A pregnant person with chlamydia can pass the infection to their baby during childbirth, which can cause eye infections or pneumonia in the newborn.
Why Most People Don’t Know They Have It
Roughly 75% of women and 50% of men with chlamydia experience no symptoms at all. This is one of the main reasons the infection spreads so effectively. People who feel perfectly healthy can unknowingly transmit it to partners for weeks or months.
When symptoms do appear, they typically show up one to three weeks after exposure. In women, signs may include unusual vaginal discharge, burning during urination, or bleeding between periods. In men, symptoms usually involve discharge from the penis, a burning sensation when urinating, or pain and swelling in one or both testicles. Rectal infections may cause discharge, pain, or bleeding regardless of sex.
What Happens If It Goes Untreated
Because chlamydia so often flies under the radar, untreated infections can cause serious damage over time. In women, the bacteria can travel upward from the cervix into the uterus and fallopian tubes, triggering pelvic inflammatory disease (PID). PID causes inflammation and scarring in the reproductive tract that may lead to chronic pelvic pain, ectopic pregnancy, or infertility. About 1 in 8 women with a history of PID have difficulty getting pregnant.
In men, untreated chlamydia can cause epididymitis, a painful inflammation of the tube that carries sperm from the testicle. While less common, this can also affect fertility. In both sexes, an untreated infection increases vulnerability to other sexually transmitted infections, including HIV.
How Chlamydia Is Detected
The standard test for chlamydia is a nucleic acid amplification test, or NAAT. This test searches for the bacterium’s genetic material in a urine sample or a swab from the cervix, urethra, rectum, or throat. NAATs detect more than 90% of chlamydia infections while maintaining specificity above 99%, meaning false positives are rare. They pick up 20% to 50% more infections than older culture-based methods could, which is why they replaced culture as the gold standard.
Testing is simple. For most people, it requires only a urine sample or a self-collected swab. Results typically come back within a few days. Because the infection is so often silent, routine screening is recommended for sexually active women under 25 and for anyone with new or multiple sexual partners.
How It’s Treated
Chlamydia is curable with antibiotics. The current first-line treatment is a seven-day course of doxycycline, taken twice daily by mouth. For people who cannot take doxycycline, a single oral dose of azithromycin is an alternative. Treatment clears the infection in the vast majority of cases, but it cannot reverse scarring or damage that has already occurred, which is why early detection matters.
You should avoid sexual contact during the full course of treatment to prevent passing the infection to a partner. Sexual partners from the preceding 60 days also need to be tested and treated, even if they have no symptoms, to prevent reinfection. Getting chlamydia once does not protect you from getting it again.