Chlamydia starts when the bacterium Chlamydia trachomatis is passed from one person to another during sexual contact, then invades the cells lining your genitals, rectum, or throat. It spreads through vaginal, anal, or oral sex without a condom with someone who carries the infection. With over 1.5 million cases reported in the U.S. in 2024 alone, it remains the most commonly reported bacterial sexually transmitted infection in the country.
How the Bacteria Gets Into Your Body
Chlamydia requires direct contact with infected mucous membranes to spread. That means the moist tissue lining the vagina, penis, rectum, or throat. The bacterium cannot survive long outside the human body, which is why you can’t pick it up from toilet seats, swimming pools, shared towels, or casual contact like hugging or shaking hands. It needs living human cells to survive and reproduce.
The three main routes of transmission are vaginal sex, anal sex, and oral sex. The infection can also spread from one site to another on the same person. For example, someone with a vaginal chlamydia infection can spread it to their rectum without having had anal sex. A pregnant person with chlamydia can also pass it to their baby during delivery, which can cause eye infections or pneumonia in the newborn.
What Happens Inside Your Cells
Once the bacterium reaches the cells lining your genital tract, throat, or rectum, it goes through a two-stage life cycle that makes it different from most other bacteria. Chlamydia is what scientists call an “obligate intracellular” organism, meaning it can only grow and reproduce inside your cells. It cannot replicate on its own the way bacteria that cause strep throat or urinary tract infections can.
The process starts with what’s known as an elementary body, a small, tough, inactive form of the bacterium designed for one purpose: attaching to the surface of a host cell and getting inside. Once the elementary body latches on, your cell essentially swallows it through a natural process where the cell membrane wraps around the invader and pulls it inward into a small bubble-like compartment.
Inside that compartment, the elementary body transforms into a larger, active form called a reticulate body. This is the stage where the bacterium starts feeding off your cell’s energy and building materials to make copies of itself. Chlamydia depends on your cells for most of the molecular fuel it needs to build proteins and genetic material. It hijacks your cell’s resources rather than producing its own. After several rounds of replication, the new bacteria convert back into elementary bodies, the tough infectious form. They then break out of the cell, either by bursting it open or by squeezing out in a packet, and go on to infect neighboring cells. This cycle repeats, spreading the infection across the tissue.
Why It Often Goes Unnoticed
One reason chlamydia is so widespread is that most people who have it don’t know. The infection frequently produces no symptoms at all, especially in its early stages. When symptoms do appear, they typically show up one to three weeks after exposure, though some people don’t notice anything for months.
In women, symptoms can include unusual vaginal discharge, burning during urination, or bleeding between periods. In men, symptoms might be a discharge from the penis, burning while urinating, or pain and swelling in one or both testicles. Rectal infections can cause pain, discharge, or bleeding regardless of sex. But many infections at all of these sites produce nothing noticeable, which is why routine screening catches most cases.
The bacterium can also enter a dormant state inside your cells when conditions aren’t ideal for it, such as when certain nutrients are limited. In this state, it’s not actively replicating but it’s still alive, which may help explain why some infections persist quietly for long periods before being detected.
Where on the Body It Takes Hold
Chlamydia doesn’t infect just one type of tissue. Different strains of the bacterium target different areas. The strains responsible for sexually transmitted infections primarily infect the cells of the cervix, urethra, rectum, and throat. A separate group of strains infects the eyes and causes trachoma, a leading infectious cause of blindness worldwide, though this is spread through direct eye contact with infected secretions rather than through sex.
Genital infections are the most common in the U.S. and other high-income countries. In 2024, reported cases were significantly higher among women (about 944,000) than men (about 564,000), though this gap partly reflects the fact that women are screened more routinely. The overall rate was roughly 446 cases per 100,000 people.
What Makes Some People More Vulnerable
The single biggest risk factor is having unprotected sex with a partner whose infection status you don’t know. Because chlamydia so often has no symptoms, even a partner who seems perfectly healthy can be carrying and transmitting the bacterium. Having multiple sexual partners increases the odds of encountering someone who is infected. Younger people, particularly those under 25, have higher rates of infection partly because of biological factors (the cervix in younger women has more of the cell type that chlamydia prefers to infect) and partly because of patterns in sexual behavior and screening.
A previous chlamydia infection does not protect you from getting it again. Reinfection is common, and each new infection carries the same risks of complications as the first, including potential damage to the reproductive tract if left untreated. Consistent condom use is the most effective way to reduce transmission risk during sex.
Why Early Infection Matters
Because chlamydia hijacks your cells quietly and efficiently, the infection can establish itself and spread through tissue well before you have any reason to suspect it. Left untreated in women, it can move from the cervix up into the uterus and fallopian tubes, causing pelvic inflammatory disease, which can lead to chronic pain, scarring, and fertility problems. In men, untreated infections can spread to the tube that carries sperm and cause pain and, rarely, affect fertility.
The good news is that chlamydia is easily curable with antibiotics once detected. The challenge is detection itself. Because the infection starts silently in most people, regular screening is the most reliable way to catch it before it causes damage. Current guidelines recommend annual screening for all sexually active women under 25, as well as for older women with new or multiple partners.