Chlamydia isn’t “formed” in the body the way a tumor or a kidney stone is. It’s caused by a bacterium called Chlamydia trachomatis that spreads from one person to another during sexual contact, then hijacks your own cells to copy itself. What makes this infection unusual is how the bacterium behaves once it’s inside you: it has a two-phase life cycle that lets it alternate between a tough, transmissible form and an active, replicating form, all while hiding from your immune system inside a protective bubble it builds within your cells.
How Chlamydia Spreads Between People
Chlamydia trachomatis targets a specific type of tissue: the columnar epithelial cells that line moist surfaces in the body. These cells are found in the cervix, urethra, rectum, and throat. The bacterium passes from person to person through direct contact with these mucosal surfaces during vaginal, anal, or oral sex. It can also pass from a pregnant person to their baby during delivery.
You don’t need to exchange bodily fluids in large quantities for transmission to happen. The infectious form of the bacterium is small enough that brief mucosal contact can be sufficient. Chlamydia doesn’t spread through casual contact like hugging, sharing food, or using the same toilet seat.
The Two Forms of the Bacterium
Most bacteria replicate by simply dividing wherever they happen to be. Chlamydia trachomatis is different. It cycles between two distinct forms, each with a specific job.
The first form is called the elementary body. This is the version that exists outside of cells and is responsible for spreading the infection. Elementary bodies are small, tough, and metabolically inactive. Think of them as the bacterium in travel mode: durable enough to survive briefly outside cells but unable to reproduce on their own. When you contract chlamydia from a partner, elementary bodies are what make the jump.
The second form is the reticulate body. Once an elementary body gets inside one of your cells, it transforms into this larger, active version. Reticulate bodies are the workhorse form. They consume nutrients from your cell and divide rapidly through simple splitting. They cannot survive outside a cell and are not infectious on their own. Late in the cycle, reticulate bodies convert back into elementary bodies, which then burst out of the cell or get released in packets to infect neighboring cells or a new host.
How the Bacterium Gets Inside Your Cells
The entry process is fast and remarkably sophisticated for such a small organism. It happens in two steps. First, the elementary body makes a loose, reversible connection with the surface of an epithelial cell, sticking to sugar-coated molecules on the cell’s outer membrane through a simple electrical attraction. This initial contact is brief because the bacterium almost immediately locks in with a stronger, permanent bond.
Once firmly attached, the bacterium deploys a needle-like injection device to shoot specialized proteins directly through the cell’s outer membrane and into its interior. These proteins hijack the cell’s own structural scaffolding, a mesh of fibers called actin that gives the cell its shape and allows it to move things around. The injected proteins force the actin network to reshape itself, creating arm-like projections on the cell surface that reach up and pull the bacterium inside. In effect, the bacterium tricks your cell into swallowing it.
Building a Safe House Inside the Cell
Once inside, the bacterium faces an immediate threat. Cells have a built-in defense system: small compartments called lysosomes that digest foreign invaders. Chlamydia avoids this by constructing a specialized compartment around itself called an inclusion. The inclusion is essentially a membrane-bound bubble that the bacterium modifies with its own proteins to prevent the cell from recognizing it as a threat and destroying it.
The inclusion does more than just provide shelter. Chlamydia inserts proteins into the inclusion’s membrane that intercept nutrient-carrying packages the cell normally sends to other destinations, redirecting them to feed the growing bacterial colony. As the reticulate bodies inside the inclusion multiply, they steal lipids and other building materials from the host cell to expand the inclusion’s membrane, making it larger over time. A mature inclusion can grow to occupy most of the interior of the host cell.
This process of copying the bacterium’s genetic material is also what drives the eventual switch from replicating forms back into infectious elementary bodies. As genome copies accumulate, the inclusion begins to divide internally into compartments, and reticulate bodies start converting back to elementary bodies in a staggered fashion, not all at once. This means the cell releases a mix of newly formed infectious particles over time rather than in a single burst.
Why Most People Don’t Notice the Infection
Chlamydia’s ability to hide inside cells and suppress the normal immune alarm system is the main reason most infections produce no symptoms. The bacterium replicates quietly within its inclusion, avoiding detection by the immune cells that patrol the mucosal surface. When symptoms do appear, they typically show up several weeks after exposure, though many people carry the infection for months without any signs at all.
This silent replication is what makes chlamydia so common. The CDC reported that cases declined 8% from 2023 to 2024, but chlamydia remains the most frequently reported bacterial sexually transmitted infection in the United States. Because most carriers don’t know they’re infected, the bacterium continues spreading through populations that aren’t being tested routinely.
What Happens if the Infection Persists
Left untreated, the cycle of cell invasion, replication, and release continues indefinitely. As more cells are destroyed and the immune system eventually mounts a stronger inflammatory response, tissue damage accumulates. In the cervix and fallopian tubes, this chronic inflammation can lead to scarring. That scarring is what causes the serious complications associated with untreated chlamydia: pelvic inflammatory disease, chronic pelvic pain, and fertility problems including ectopic pregnancy.
In the urethra, persistent infection can cause painful urination and discharge. In the rectum, it can cause discomfort and bleeding. The damage isn’t from the bacterium directly destroying large amounts of tissue. It’s from your own immune system’s prolonged inflammatory response to an infection it can detect but can’t fully clear on its own without help.
The good news is that antibiotics are highly effective at eliminating chlamydia once it’s diagnosed. The infection is curable, and treatment prevents the scarring that leads to long-term complications. The challenge is catching it, which is why routine screening is the most effective tool for controlling its spread.