Chlamydia isn’t “created” in the body the way some infections can arise from normal bacteria going rogue. It’s caused by a specific bacterium, Chlamydia trachomatis, that can only be passed from one person to another. You can’t develop it spontaneously, and it doesn’t come from toilet seats, swimming pools, or casual contact. Every chlamydia infection traces back to sexual contact with someone who already carries the bacteria.
Where the Bacteria Came From
Chlamydia trachomatis has been evolving alongside humans for an extraordinarily long time. It shared a common ancestor with environmental chlamydiae roughly 700 million years ago. Over that span, the bacterium became completely dependent on human cells to survive and reproduce. Its genome shrank dramatically as it lost the ability to live independently, shedding genes it no longer needed because it could steal nutrients and energy directly from the cells it infects.
This is why chlamydia can’t survive on surfaces, in water, or in the open air for any meaningful length of time. It’s an obligate intracellular parasite, meaning it literally cannot reproduce outside of a living human cell. That total dependence on a host is also why the only way to get it is through direct contact with infected mucous membranes during vaginal, anal, or oral sex.
How the Bacteria Infects a Cell
The chlamydia lifecycle has two distinct forms. The first is a tough, compact particle designed to survive briefly outside cells and latch onto a new one. Once this particle attaches to cells lining the genitals, rectum, or throat, it triggers those cells to pull it inside, almost like tricking the cell into swallowing it.
Within about 6 to 8 hours, the particle transforms into a larger, softer form that can feed and divide. It builds a protective bubble inside the host cell called an inclusion, which acts like a private apartment. The bacterium uses a needle-like injection system to pump proteins through the wall of this bubble and into the surrounding cell. These proteins hijack the cell’s own supply lines, redirecting fats, calcium, and other nutrients into the inclusion to fuel bacterial growth.
Between roughly 8 and 16 hours, the bacteria begin dividing rapidly and the inclusion swells. By 24 to 72 hours after the initial infection, some of the dividing bacteria start converting back into the tough, compact form. Eventually the inclusion either ruptures the host cell or pinches off, releasing hundreds of new infectious particles that spread to neighboring cells or get passed to a sexual partner.
Why Most People Don’t Know They Have It
One of the reasons chlamydia spreads so effectively is that the majority of infected people feel perfectly fine. About 70 to 80% of women and up to 50% of men with chlamydia have no symptoms at all. When symptoms do appear, they typically show up one to three weeks after exposure and can include unusual discharge, burning during urination, or, in women, bleeding between periods.
Because the infection is silent in most cases, people unknowingly pass it to partners for weeks or months. This is the primary mechanism behind chlamydia’s persistence as the most commonly reported bacterial sexually transmitted infection in the United States, with chlamydia contributing to the more than 2.2 million combined cases of chlamydia, gonorrhea, and syphilis reported in 2024.
What Happens If It Goes Untreated
Left alone, chlamydia doesn’t resolve quickly or harmlessly in everyone. In women, the infection can climb from the cervix into the uterus and fallopian tubes, causing pelvic inflammatory disease. The overall risk of developing PID after a chlamydia infection is roughly 15 to 17%, including cases that go undiagnosed because the inflammation is too mild to cause obvious pain. PID can scar the fallopian tubes, leading to chronic pelvic pain, ectopic pregnancy, or infertility.
In men, untreated chlamydia can cause inflammation of the tube that carries sperm, which brings pain and swelling in the testicles and, rarely, affects fertility. Both men and women can develop reactive arthritis, a condition where the immune response to the infection triggers joint pain, eye inflammation, and urinary symptoms that can persist for months.
How It’s Detected and Treated
Modern testing uses a method that amplifies tiny traces of the bacterium’s genetic material from a urine sample or swab. These tests are highly accurate, with about 94% sensitivity and 99% specificity, meaning they catch nearly all true infections while almost never producing a false positive. Results typically come back within a few days.
Treatment is straightforward. A standard course of oral antibiotics taken for seven days clears the infection in the vast majority of cases. Because reinfection is common if a partner isn’t also treated, both partners need to complete treatment and avoid sexual contact until the course is finished. Retesting about three months later is recommended to confirm the infection hasn’t returned through re-exposure.
Why It Keeps Spreading
Chlamydia persists not because it’s hard to treat but because it’s hard to detect without routine screening. The combination of a short, silent incubation period, a high rate of asymptomatic infection, and the bacterium’s efficient cell-to-cell lifecycle creates a cycle where people spread it before they have any reason to get tested. Consistent condom use significantly reduces transmission risk, and annual screening for sexually active women under 25 (the highest-risk group) is the most effective public health tool for breaking the chain of silent transmission.