Chlamydia trachomatis is a species of bacteria responsible for chlamydia, the most commonly reported sexually transmitted infection in the United States. In 2024, over 1.5 million cases were reported to the CDC. What makes this bacterium particularly effective at spreading is that roughly 75% of infected women and 50% of infected men never develop symptoms, meaning millions of people carry and transmit it without knowing.
How Chlamydia Trachomatis Works
Chlamydia trachomatis is an unusual bacterium. Unlike most bacteria that can survive on their own, it’s an obligate intracellular pathogen, meaning it can only live and reproduce inside human cells. It lacks the ability to produce its own energy and depends on the host cell for fuel, including ATP (the molecule cells use as energy currency). This is why chlamydia can’t survive on surfaces like toilet seats or doorknobs.
The bacterium has a two-phase lifecycle that alternates between two forms. The first is the elementary body, a tiny, tough particle about 0.25 to 0.3 micrometers across. This is the infectious form that passes between people. Once an elementary body enters a human cell, it transforms into a larger reticulate body (0.5 to 0.6 micrometers) that can divide and multiply. After roughly 20 hours of development, the reticulate bodies begin producing new elementary bodies, which eventually burst out of the host cell and spread to infect neighboring cells or a new person.
Different Strains Cause Different Diseases
Not all strains of Chlamydia trachomatis do the same thing. The species is divided into groups called serovars, and each group targets different parts of the body.
- Serovars A through C cause trachoma, an eye infection that remains the world’s leading infectious cause of blindness. This is spread through contact with eye secretions, not sexual activity.
- Serovars D through K are responsible for the common genital and urinary tract infections most people associate with “chlamydia.” These strains can also infect the eyes and rectum.
- Serovars L1 through L3 cause lymphogranuloma venereum (LGV), a more aggressive sexually transmitted infection that invades the lymph nodes and requires a longer course of treatment (21 days compared to the standard 7).
When someone in the U.S. is diagnosed with chlamydia, it’s almost always a serovar D through K infection.
How It Spreads
Chlamydia trachomatis passes from person to person through vaginal, anal, or oral sex without a condom. The bacterium can infect the cervix, urethra, rectum, and throat. Rectal infections can happen through receptive anal sex or by spreading from another infected site, like the vagina.
A pregnant woman with chlamydia can also pass the infection to her baby during childbirth. This can cause eye infections or pneumonia in the newborn.
Symptoms and the Silence Problem
The biggest challenge with chlamydia is that most infections produce no obvious signs. Three out of four women and half of men who are infected feel completely fine. This is why chlamydia is sometimes called a “silent” infection.
When symptoms do appear, they typically show up one to three weeks after exposure. Women may notice unusual vaginal discharge, burning during urination, or bleeding between periods. Men may experience discharge from the penis, burning with urination, or pain and swelling in one testicle. Rectal infections in either sex can cause discharge, pain, or bleeding, though many rectal infections are also symptom-free.
What Happens If It Goes Untreated
Because so many people don’t realize they’re infected, chlamydia often goes untreated for months or even years. This is where serious damage can occur. About 10 to 15% of women with untreated chlamydia develop pelvic inflammatory disease (PID), an infection of the uterus, fallopian tubes, and surrounding tissue. PID can cause chronic pelvic pain, and the scarring it leaves behind in the fallopian tubes can lead to infertility or ectopic pregnancy, where a fertilized egg implants outside the uterus.
Even without progressing to full PID, chlamydia can cause what’s known as “silent” infection in the upper reproductive tract, quietly damaging the fallopian tubes without producing noticeable symptoms. In men, untreated chlamydia can lead to infection of the tube that carries sperm (epididymitis), which causes pain and can, in rare cases, affect fertility.
How Chlamydia Is Diagnosed
The standard test for chlamydia is a nucleic acid amplification test (NAAT), which detects the bacterium’s genetic material. NAATs are highly accurate, with sensitivity usually above 90% and specificity of 99% or higher, meaning false positives are rare and the test catches the vast majority of infections. Testing typically involves a urine sample or a swab from the cervix, urethra, rectum, or throat, depending on the sites of potential exposure.
Because chlamydia so often lacks symptoms, routine screening is essential for catching infections early. The CDC recommends annual chlamydia screening for all sexually active women under 25, and for women 25 and older with risk factors like new or multiple partners. Sexually active men who have sex with men should be screened at least annually at all sites of contact, with more frequent testing (every 3 to 6 months) for those at higher risk. Transgender and gender diverse individuals should be screened based on their anatomy and sexual practices. All pregnant women under 25 should be screened, with retesting in the third trimester.
Treatment
Chlamydia is curable with antibiotics. The current first-line treatment is a seven-day course of doxycycline taken twice daily. A once-daily delayed-release formulation of the same antibiotic is equally effective and may be easier to stick with. For people who can’t take doxycycline, a single dose of azithromycin is an alternative, though it’s no longer the preferred option.
After finishing treatment, retesting about three months later is recommended because reinfection is common, especially if a sexual partner wasn’t treated at the same time. Pregnant women should be retested even sooner, four weeks after completing treatment, to confirm the infection has cleared.
Who Gets Chlamydia
In 2024, roughly 944,000 cases were reported among women and 564,000 among men in the United States. Women are diagnosed at higher rates partly because they’re screened more routinely, not necessarily because they’re infected more often. The overall rate was about 446 cases per 100,000 people. Cases actually declined 8% from 2023, continuing a two-year downward trend, though reported numbers still undercount the true burden given how many infections go undetected.
Young age is the single strongest risk factor. The highest rates consistently fall among 15- to 24-year-olds. Other factors that increase risk include having a new sexual partner, multiple partners, inconsistent condom use, a partner with concurrent sexual relationships, or a previous STI.