Chlamydia is caused by a bacterium called Chlamydia trachomatis, and it spreads primarily through vaginal, anal, or oral sex without a condom. 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. Understanding exactly how the bacterium works, how it spreads, and what makes certain people more vulnerable can help you protect yourself and recognize when to get tested.
The Bacterium Behind the Infection
Chlamydia trachomatis is a tiny, unusual bacterium. Unlike most bacteria that can survive on their own, it can only live and reproduce inside human cells. This makes it an obligate intracellular parasite, meaning it hijacks your own cells to complete its life cycle.
The bacterium exists in two forms. The first is a small, tough particle designed to survive outside cells and latch onto new ones. Once this particle attaches to the thin layer of cells lining your genital tract, throat, or rectum, it enters the cell and transforms into a second, larger form that can rapidly divide and multiply. Over the next 48 to 72 hours, the bacteria replicate until the host cell bursts open, releasing a new wave of infectious particles that go on to infect neighboring cells. This cycle of invasion, multiplication, and cell destruction is what drives the infection forward and, over time, causes tissue damage.
How Chlamydia Spreads
The primary route of transmission is sexual contact. You can get chlamydia through vaginal, anal, or oral sex with someone who carries the infection, even if that person has no symptoms. The bacterium lives in infected fluids and on mucous membranes, so any unprotected contact with these surfaces can transfer it.
Rectal infections can happen through receptive anal sex, but the bacteria can also spread from one infected site to another on the same person. For example, a vaginal infection can spread to the rectum without anal sex being involved. A pregnant person with chlamydia can also pass the infection to their baby during vaginal delivery, which can cause eye infections or pneumonia in the newborn.
Chlamydia does not spread through casual contact like hugging, sharing food, or sitting on toilet seats. It requires direct contact with infected mucous membranes or fluids.
Where the Infection Takes Hold
Chlamydia trachomatis targets the columnar epithelial cells that line certain parts of the body. In women, it most commonly infects the cervix. In men, it targets the urethra. Both men and women can develop infections in the rectum or throat depending on the type of sexual contact involved.
In some women, what starts as a straightforward cervical infection can silently move upward into the uterus and fallopian tubes. This ascending infection can lead to pelvic inflammatory disease, which causes scarring in the reproductive tract. That scarring is the mechanism behind chlamydia-related infertility and ectopic pregnancies. The progression can happen without noticeable symptoms, which is why screening matters even when you feel fine.
Why Most People Don’t Know They Have It
One of the most important things about chlamydia is that it usually produces no symptoms at all. Roughly 75% of women and 50% of men with the infection are completely asymptomatic. This is a direct consequence of how the bacterium operates: it replicates inside cells relatively quietly, and the immune response it triggers is often too mild to produce obvious signs, at least in the early stages.
When symptoms do appear, they typically show up one to three weeks after exposure. Women may notice unusual vaginal discharge or burning during urination. Men may experience discharge from the penis or testicular discomfort. Rectal infections can cause pain, discharge, or bleeding. But because the majority of infections are silent, relying on symptoms alone means most cases go undetected and untreated, allowing the bacteria to cause damage over months or years and spread to sexual partners unknowingly.
Who Is Most at Risk
Certain factors make some people significantly more likely to contract chlamydia. Age is the single strongest predictor. Prevalence is highest among adolescents and young adults under 25. This is partly biological: younger women are more likely to have a type of cervical tissue (where the inner cervical cells extend onto the outer surface of the cervix) that is especially vulnerable to the bacterium. It’s also behavioral and contextual. Younger people are statistically more likely to have new or multiple sexual partners, and some may have less ability to negotiate condom use in their relationships.
The numbers bear this out. Among sexually active people who reported just one partner in the past year, the infection rate was about 1.4%. Among those with two or more partners, that rate jumped to 3.2%, more than double. Other demographic factors linked to higher prevalence include lower income, lower education level, and being unmarried. Notably, using hormonal birth control like the pill or injectable contraceptives does not appear to increase or decrease the risk of infection itself, though these methods obviously don’t provide a physical barrier against the bacterium the way condoms do.
How It Differs From Other STIs
Chlamydia is sometimes confused with gonorrhea because the two infections share similar symptoms and can infect the same sites. The key difference is the bacterium involved: gonorrhea is caused by a completely different species. Both can be present at the same time, which is why testing for one often includes testing for the other.
Unlike viral STIs such as herpes or HIV, chlamydia is fully curable with antibiotics. A standard course of treatment clears the infection in the vast majority of cases. However, being cured does not make you immune. You can be reinfected every time you’re exposed, and repeated infections increase the cumulative risk of complications like pelvic inflammatory disease and fertility problems. This is why retesting a few months after treatment is recommended if you remain sexually active.
The Role of Screening
Because chlamydia is so often silent, routine screening is the most effective way to catch it early. The CDC recommends annual chlamydia screening for all sexually active women under 25, as well as for older women with risk factors like new or multiple partners. Testing is simple, typically requiring only a urine sample or a swab, and results come back quickly.
For men, routine screening is less universally recommended but is advised for those with multiple partners or those who have sex with men. Rectal and throat testing is available for anyone who may have been exposed at those sites, though these tests need to be specifically requested since standard screening usually only checks the genital tract.