Chlamydia is caused by infection with the bacterium Chlamydia trachomatis, spread primarily through vaginal, anal, or oral sexual contact. It is the most commonly reported bacterial sexually transmitted infection in the United States, contributing to over 2.2 million combined STI cases reported in 2024. The bacterium targets a specific type of cell lining the genital tract, which is why it spreads so efficiently through sexual activity and why certain people are more biologically vulnerable than others.
The Bacterium Behind the Infection
Chlamydia trachomatis is not like most bacteria. It cannot survive or reproduce on its own. It is an obligate intracellular pathogen, meaning it must get inside your cells to live. Specifically, it targets columnar epithelial cells, the thin, column-shaped cells that line moist surfaces like the cervix, urethra, rectum, and throat. In women, the endocervix (the inner part of the cervix) is the most commonly infected site. From there, the bacteria can sometimes travel upward into the uterine lining and fallopian tubes.
How the Bacteria Invade Your Cells
Chlamydia has an unusual two-stage life cycle that makes it especially effective at spreading. The bacterium alternates between two forms: an elementary body (EB) and a reticulate body (RB). Each plays a distinct role.
Elementary bodies are tiny, roughly 0.3 micrometers across, and are metabolically inactive. Think of them as a durable, spore-like package designed for one purpose: attaching to a new cell and getting inside. Once an EB latches onto one of your epithelial cells, it gets pulled inside a small bubble called an inclusion, which shields it from your immune system.
Inside that protective bubble, the EB transforms into a reticulate body. RBs are larger (about 1 micrometer), metabolically active, and capable of dividing rapidly. They multiply through repeated rounds of cell division, all while hiding inside your own cell. After this growth phase, the RBs convert back into elementary bodies. Eventually the host cell bursts open, releasing a fresh wave of infectious EBs that go on to infect neighboring cells. This cycle is why a chlamydia infection can persist and spread through tissue even while causing few or no symptoms.
How Chlamydia Spreads Between People
The primary route is sexual contact. Chlamydia passes through infected genital fluids during vaginal, anal, or oral sex. You do not need to experience ejaculation for transmission to occur. Any direct contact between infected mucous membranes is enough.
Transmission probability varies. Modeling studies estimate that during a single sexual partnership, the chance of male-to-female transmission is roughly 32 to 35 percent. Female-to-male transmission appears lower, with estimates ranging widely from about 5 to 21 percent depending on the study population. Per individual sex act, estimates range from about 4 to 15 percent. These numbers help explain why chlamydia spreads so readily: even a single unprotected encounter carries meaningful risk, and the risk compounds over the course of a relationship.
Mother-to-Child Transmission
Chlamydia can also pass from mother to baby, and the rates are significant. Between 50 and 70 percent of newborns acquire the infection from an untreated mother. The traditional assumption was that babies pick up the bacteria while passing through the birth canal, but research has shown that nearly half of infected newborns in one study were delivered by cesarean section. This suggests the bacteria can reach the fetus before delivery, potentially through the placenta, amniotic fluid, or the membranes surrounding the fetus.
For affected newborns, the nasopharynx (the area behind the nose) is the most frequently infected site. About 30 to 50 percent of infected babies develop conjunctivitis (eye infection) or pneumonia. Infected newborns have a pneumonia risk 3.6 times higher than uninfected babies. Treatment of the mother during pregnancy dramatically reduces vertical transmission, dropping rates from about 30 percent to just 1.5 percent in one cohort study.
Why Young Women Are More Vulnerable
Chlamydia disproportionately affects women and adolescents under 20. One important reason is biological. In younger women, the cervix has more columnar epithelial cells exposed on its outer surface, a normal developmental feature called cervical ectopy. Since these are exactly the cells chlamydia targets, a younger cervix essentially presents more entry points for the bacterium. As women age, these cells gradually recede inward, reducing the exposed surface area.
The numbers reflect this vulnerability clearly. In one study of pregnant women, the infection rate was 11.6 percent among women under 25 compared to just 2 percent in those over 25.
Can You Catch It From Surfaces?
This is a common concern, and the short answer for genital chlamydia is that surface transmission is not a recognized route. Chlamydia trachomatis is fragile outside the body. Research on the trachoma strains of the same bacterium (which cause eye infections, not genital infections) found that viable bacteria could be recovered from plastic, skin, and cotton cloth for up to 24 hours, but only when deposited in relatively high concentrations. On woven mats, the bacteria did not survive at all. Cotton cloth showed the fastest decline.
These findings relate to eye-to-surface-to-eye transmission of trachoma in specific settings, not to genital infections. You are not going to contract genital chlamydia from a toilet seat, shared towel, or swimming pool. The bacterium requires direct contact with infected mucous membranes or fluids to establish a genital infection.
Why It Often Goes Undetected
One of the most important things about how chlamydia establishes itself is how quietly it does so. If symptoms appear at all, they typically don’t show up until several weeks after exposure. But the majority of infections produce no noticeable symptoms. This silent nature is a major reason chlamydia spreads so effectively: people who don’t know they’re infected continue having sex and unknowingly pass the bacteria along. It also means the infection can persist long enough for the bacteria to migrate to the upper reproductive tract in women, where it can cause scarring in the fallopian tubes and lead to complications like pelvic inflammatory disease or fertility problems.