Chlamydia trachomatis is a species of bacteria that causes chlamydia, the most common bacterial sexually transmitted infection in the world. In the United States alone, over 1.5 million cases were reported in 2024. The bacterium is unusual because it can only survive and reproduce inside human cells, which is why it was originally mistaken for a virus. It targets the mucous membranes of the genitals, rectum, and eyes, and it often causes no symptoms at all, making it easy to spread without either partner knowing.
How C. Trachomatis Differs From Other Bacteria
Most bacteria can live independently, reproducing on surfaces, in water, or in soil. C. trachomatis cannot. It is an obligate intracellular parasite, meaning it must get inside a human cell to survive and multiply. This dependence on a host cell is so extreme that early researchers classified it as a virus. It was reclassified as a bacterium once scientists confirmed it has its own cell wall, DNA, RNA, and ribosomes, all hallmarks of bacterial life.
The bacterium is tiny and nonmotile (it can’t propel itself). It belongs to the order Chlamydiales and exists in a single genus, Chlamydia. What makes it medically significant is not just that it infects human cells, but that it has evolved a two-stage life cycle specifically designed to spread between people and then hijack the cell’s own energy supply once inside.
The Two-Stage Life Cycle
C. trachomatis alternates between two distinct forms. The first, called the elementary body, is the infectious form that survives outside the body. It has a tough, spore-like outer wall stabilized by cross-linked proteins, giving it resistance to physical and environmental stress. This is the form that passes between people during contact.
Once an elementary body lands on a mucosal surface, it enters a cell and settles into a protective bubble called an inclusion. Within about 6 to 8 hours, it transforms into the second form: the reticulate body. This version can’t survive outside a cell, but it’s built for one job: replication. Reticulate bodies are metabolic powerhouses, equipped with machinery for generating energy, building proteins, and pulling nutrients from the host cell. Between roughly 8 and 16 hours after infection, they begin dividing rapidly, and the inclusion swells inside the cell.
At around 24 to 72 hours, the cycle reverses. Reticulate bodies begin converting back into elementary bodies in staggered waves. Eventually the host cell ruptures or releases the elementary bodies, which go on to infect neighboring cells or get transmitted to a new person. This cycle is why chlamydia can establish a persistent infection if left untreated.
Three Groups of Serovars, Three Types of Disease
Not all C. trachomatis infections look the same because the species contains multiple serovars (subtypes), each with slightly different surface proteins that determine which tissues they target and how severe the infection becomes.
- Serovars A, B, Ba, and C cause trachoma, a chronic eye infection found primarily in developing countries. Repeated infections over years scar the inside of the eyelid, eventually turning the eyelashes inward so they scrape the cornea. This leads to irreversible corneal damage and blindness. Trachoma remains a public health problem in 30 countries and has caused blindness or visual impairment in about 1.9 million people worldwide.
- Serovars D through K are responsible for the genital and urinary tract infections most people associate with chlamydia. These are the serovars behind the vast majority of reported cases in the U.S. and other high-income countries.
- Serovars L1, L2, and L3 cause lymphogranuloma venereum (LGV), a more invasive sexually transmitted infection that penetrates deeper tissue and inflames the lymph nodes, particularly in the groin or rectum.
Symptoms in Women, Men, and Rectal Infections
Chlamydia is often called a “silent” infection. The majority of people who have it notice nothing unusual, which is the primary reason it spreads so effectively. When symptoms do appear, they differ by site.
Women with symptoms typically notice an abnormal vaginal discharge or a burning sensation when urinating. Even without symptoms, the infection can silently damage reproductive tissue. Men are more likely to notice discharge from the penis, burning during urination, or, less commonly, pain and swelling in one or both testicles.
Rectal infections occur in both men and women and often produce no symptoms. When they do, symptoms include rectal pain, discharge, and bleeding.
What Happens If It Goes Untreated
The real danger of C. trachomatis is what it does over time without treatment. Roughly 20% of women with a lower genital tract chlamydia infection will develop pelvic inflammatory disease (PID), a condition where the infection spreads to the uterus, fallopian tubes, or surrounding tissue. PID can cause chronic pelvic pain in about 4% of those infected and infertility in about 3%, typically because scar tissue blocks the fallopian tubes. Around 2% experience adverse pregnancy outcomes, including miscarriage and stillbirth.
In men, untreated chlamydia can inflame the tube that carries sperm from the testicle, causing pain and, in rare cases, affecting fertility. Regardless of sex, an active chlamydia infection also increases vulnerability to other sexually transmitted infections.
How It’s Diagnosed
The standard test for C. trachomatis is a nucleic acid amplification test (NAAT), which detects the bacterium’s genetic material in a urine sample or swab. NAATs have sensitivity usually well above 90% and specificity of 99% or higher, meaning false negatives are uncommon and false positives are rare. This replaced the older method of trying to grow the bacteria in a lab culture, which was slower and less sensitive. Testing is simple: for most people it involves either a urine sample or a quick swab, with results typically available within a few days.
Treatment and Recovery
C. trachomatis is curable with antibiotics. The current first-line treatment is a week-long course of an oral antibiotic taken twice daily. An alternative is a single-dose oral antibiotic for people who may have difficulty completing the full course. Most people clear the infection completely with treatment, but reinfection is common if sexual partners are not treated at the same time. You can be reinfected immediately after finishing treatment if exposed again, so retesting a few months later is a standard recommendation.
Who Gets Infected Most Often
In 2024, provisional U.S. data showed about 1.52 million reported chlamydia cases, with women accounting for roughly 944,000 and men about 564,000. The gap partly reflects higher screening rates among women rather than a true difference in infection rates. Sexually active people under 25 carry the highest burden, which is why annual screening is recommended for that age group. The overall case count dropped 8% from 2023, continuing a two-year decline, though underreporting remains likely given how often the infection produces no symptoms.
Trachoma and Global Blindness
Outside of sexually transmitted disease, C. trachomatis is also the leading infectious cause of preventable blindness. Trachoma spreads through contact with eye or nose secretions, often among young children in communities with limited access to clean water and sanitation. A single episode of eye infection causes inflammation but not lasting damage. The problem is repeated reinfection over years, which progressively scars the inner eyelid until the eyelashes turn inward and abrade the cornea with every blink. This constant friction eventually clouds the cornea permanently.
About 97 million people currently live in trachoma-endemic areas and are at risk. The disease accounts for roughly 1.4% of all blindness worldwide. Mass antibiotic distribution, facial cleanliness campaigns, and surgical correction of inward-turning eyelashes have eliminated trachoma in many countries, but it persists in 30 nations, mostly in sub-Saharan Africa and parts of Asia.