Chlamydia spreads primarily through vaginal, anal, or oral sex with someone who has the infection. It is the most commonly reported bacterial sexually transmitted infection in the United States, with over 1.5 million cases reported in 2024 alone. Because the majority of infected people have no symptoms, many pass it on without ever knowing they carry it.
Sexual Contact Is the Primary Route
Chlamydia transmits through direct contact with infected genital, anal, or oral fluids during sex. This includes vaginal intercourse, anal intercourse, and oral sex (both giving and receiving). The bacteria infect the mucous membranes of the cervix, urethra, rectum, and throat, meaning any unprotected sexual contact involving these areas creates an opportunity for transmission. You don’t need to have penetrative sex to be exposed. Any genital-to-genital contact or genital-to-mouth contact that involves infected fluids can carry risk.
Ejaculation doesn’t have to occur for the infection to spread. Simply having unprotected contact with an infected area is enough. The bacteria live in the cells lining the reproductive tract, rectum, and throat, so they’re present in the fluids and on the surfaces of those tissues during any sexual activity.
How the Bacteria Actually Infect You
Once chlamydia reaches a mucous membrane, the infection process happens at the cellular level. The bacteria exist in a tiny, dormant form (roughly 0.3 micrometers, far smaller than most bacteria) that attaches to the surface of your cells in a two-step process. First, it loosely binds to sugar molecules on the cell surface. Then it locks on more firmly and injects a signaling protein into the cell that forces your cell to pull the bacterium inside.
Once inside, the bacteria switch into an active, growing form. They multiply repeatedly within a protective bubble inside your cell, then convert back into their infectious dormant form. Eventually the host cell bursts open, releasing hundreds of new infectious particles that spread to neighboring cells. This cycle is why untreated chlamydia can persist for months or longer and why a single exposure can establish a lasting infection.
Most People With Chlamydia Have No Symptoms
This is the most important thing to understand about how chlamydia spreads: the vast majority of carriers don’t know they’re infected. About 75% of women and 50% of men with chlamydia experience no symptoms at all. That means a sexual partner can look and feel perfectly healthy while carrying and transmitting the infection.
When symptoms do appear, they typically show up 7 to 21 days after exposure, though in some cases it can take several months. Common signs include unusual discharge, burning during urination, or pain during sex. But because most infections are silent, routine screening is the only reliable way to catch them. This asymptomatic spread is the main reason chlamydia remains so widespread, even as overall case numbers have declined in recent years.
Throat and Eye Infections
Chlamydia can infect the throat through oral sex, particularly when giving oral sex to a partner with a genital or rectal infection. Pharyngeal (throat) chlamydia often causes no symptoms and may be less harmful than a genital infection, but having chlamydia in the throat can make it easier to spread the bacteria to others during oral sex.
Eye infections are also possible. These happen through autoinoculation, meaning you transfer the bacteria from an infected genital area to your eye by touching infected fluids and then touching your eye. The American Academy of Ophthalmology notes that transmission occurs through direct contact of the eye with infected urinary or genital fluids. This is uncommon but worth knowing about, since chlamydial eye infections can cause significant irritation and, if untreated, potential vision problems.
Transmission During Childbirth
A pregnant person with an untreated chlamydia infection can pass the bacteria to their baby during vaginal delivery. As the newborn moves through the birth canal, it comes into direct contact with infected tissue. This can lead to eye infections (neonatal conjunctivitis) or pneumonia in the infant, both of which can be serious. This is one reason prenatal screening for chlamydia is standard practice during pregnancy.
Ways You Cannot Get Chlamydia
Chlamydia bacteria cannot survive outside the human body for more than a very short time. You cannot get chlamydia from toilet seats, swimming pools, hot tubs, shared towels, doorknobs, or casual contact like hugging or shaking hands. The bacteria require the warm, moist environment of human mucous membranes to survive and reproduce. On hard surfaces like a toilet seat, they die quickly.
You also cannot get chlamydia from kissing, sharing food or drinks, or breathing the same air as an infected person. It is not airborne and does not spread through saliva under normal circumstances. The only meaningful routes of transmission are sexual contact and passage through the birth canal.
Who Is Most at Risk
Chlamydia rates are highest among people ages 15 to 24, and women are diagnosed at higher rates than men. In 2024, the reported rate among women in the U.S. was about 550 cases per 100,000 people, compared to 335 per 100,000 among men. Part of this difference reflects the fact that women are screened more frequently, but biological factors also play a role: the cervix is particularly susceptible to chlamydial infection in younger women.
Your risk increases with each new sexual partner and with inconsistent condom use. Having had chlamydia before does not protect you from getting it again. Reinfection is common, especially if a partner was not treated at the same time. Using condoms consistently and correctly during vaginal, anal, and oral sex significantly reduces your risk, though it doesn’t eliminate it entirely since skin-to-skin contact around areas not covered by a condom can still transmit the bacteria in some cases.
How Long You’re Infectious
Without treatment, a chlamydia infection can persist for months or even longer, and you remain capable of transmitting the bacteria the entire time. Because the incubation period ranges from about one to three weeks (and sometimes longer), there’s a window after exposure during which you’re infected and potentially infectious but wouldn’t yet test positive. Most guidelines recommend testing at least one to two weeks after a potential exposure for the most accurate results, though your healthcare provider can advise on timing based on your specific situation.
After completing treatment, it takes about seven days before the infection clears enough to avoid passing it to a partner. Retesting three months after treatment is recommended, since reinfection rates are high, particularly if your sexual partner wasn’t treated simultaneously.