Chlamydia is caused by a bacterium called Chlamydia trachomatis, and it spreads through sexual contact. It is the most commonly reported bacterial sexually transmitted infection in the United States, with over 1.5 million cases recorded in 2024 alone. Understanding the specific bacterium behind the infection, how it gets into the body, and why it so often goes unnoticed helps explain why chlamydia remains so widespread.
The Bacterium Behind the Infection
Chlamydia trachomatis is a tiny bacterium that cannot survive on its own. Unlike bacteria that can live on surfaces or in soil, this one is an obligate intracellular pathogen, meaning it must get inside human cells to reproduce. Over a very long evolutionary history, the bacterium shed much of its own genetic material and became completely dependent on the energy and resources of the cells it infects.
What makes this bacterium unusual is its two-phase life cycle, which takes roughly 48 to 72 hours to complete. It exists in two distinct forms. The first is a small, tough, dormant form that can survive outside of cells. This is the version that passes between people during sex. Once it attaches to the lining of the cervix, urethra, rectum, or throat, it tricks the cell into pulling it inside. There, it transforms into a larger, active form that rapidly divides. After enough copies have been made, the new bacteria convert back into the small infectious form and burst out of the cell, ready to infect neighboring cells or spread to a new person.
The bacterium attaches to human cells using a sugar-based molecule on its outer surface that acts like a bridge, linking it to matching molecules on the host cell. This attachment step is essential. Without it, the bacterium cannot enter a cell and cannot cause infection.
How Chlamydia Spreads
Chlamydia is transmitted through vaginal, anal, or oral sex without a condom with someone who carries the infection. It does not spread through casual contact like hugging, sharing food, or using the same toilet seat. The bacterium lives in the mucous membranes of the genitals, rectum, and throat, and requires direct contact with these tissues to transfer.
Rectal infections can happen through receptive anal sex or through the spread of bacteria from another infected site, such as the vagina. A pregnant person with chlamydia can also pass the bacterium to their baby during vaginal delivery, which can cause eye infections or pneumonia in the newborn.
Reinfection is common. Having chlamydia once does not protect you from getting it again. If your partner is not treated at the same time, the infection can simply pass back and forth.
Why Most People Don’t Know They Have It
The single biggest reason chlamydia spreads so effectively is that it usually causes no symptoms at all. About 75 percent of women and 50 percent of men with the infection never notice anything wrong. A person can carry and transmit chlamydia for weeks or months without any sign of illness, which is why routine screening is the primary way infections get caught.
When symptoms do appear, they typically show up one to three weeks after exposure. In women, this might look like unusual vaginal discharge, a burning feeling during urination, or bleeding between periods. In men, symptoms can include discharge from the penis, burning with urination, or pain and swelling in one or both testicles. Rectal infections may cause discharge, pain, or bleeding regardless of sex.
Who Is Most at Risk
Anyone who is sexually active can get chlamydia, but certain factors raise the likelihood. Having unprotected sex is the most significant risk factor. Having multiple sexual partners increases the number of potential exposures. Younger age also plays a role: rates are highest among people in their late teens and twenties, partly because of biological factors (the cervix in younger women is more susceptible to infection) and partly because of higher rates of partner change in that age group.
Women are diagnosed nearly twice as often as men. In 2024, the reported rate was about 550 cases per 100,000 women compared to 335 per 100,000 men. Some of this gap reflects the fact that women are screened more regularly, but biological vulnerability also contributes.
What Happens If Chlamydia Goes Untreated
Left alone, the bacterium does not just sit quietly. In women, the infection can climb from the cervix into the uterus and fallopian tubes, causing pelvic inflammatory disease (PID). PID triggers inflammation that leads to scar tissue forming inside and around the fallopian tubes. That scarring can block the tubes entirely, leading to infertility or ectopic pregnancy, where a fertilized egg implants outside the uterus. It can also cause chronic pelvic pain that persists long after the infection itself has been treated. The longer the infection goes untreated, the greater the chance of these complications.
In men, untreated chlamydia can spread to the tube that carries sperm from the testicle, causing pain and swelling. In rare cases, this can affect fertility. Rectal infections, if left untreated, can cause ongoing discomfort and inflammation.
How Treatment Works
Because chlamydia is bacterial, it is curable with antibiotics. Treatment is straightforward and typically involves a short course of oral medication. Symptoms, if present, usually clear within a week or two. Both you and any recent sexual partners need to be treated to prevent passing the infection back and forth. Retesting about three months after treatment is recommended, since reinfection rates are high.
The good news reflected in recent surveillance data: chlamydia cases in the U.S. declined for the second year in a row in 2024, dropping 8 percent from the previous year. Whether that trend continues depends largely on screening rates, condom use, and prompt treatment of infections before they have a chance to spread.