Chlamydia spreads through sexual contact with an infected person. It is the most commonly reported bacterial sexually transmitted infection in the United States, with over 1.5 million cases recorded in 2024 alone. The tricky part: 75% of infected women and 50% of infected men have no symptoms at all, which means many people pass it on without ever knowing they have it.
Sexual Contact Is the Primary Route
Chlamydia is caused by the bacterium Chlamydia trachomatis, and it transmits through vaginal, anal, or oral sex with someone who carries the infection. The bacteria live in infected bodily fluids, including semen and vaginal fluid, and target the moist, mucous membrane surfaces of the body. That means the genitals, rectum, and throat are all potential infection sites depending on the type of sexual contact involved.
You don’t need to have penetrative sex to get chlamydia. Any genital-to-genital contact or contact between genitals and the mouth or rectum can transfer the bacteria. Sharing sex toys without washing them or using a new condom between partners is another possible route. Ejaculation doesn’t have to occur for the infection to spread.
Importantly, you can get chlamydia more than once. A previous infection does not give you any immunity. If you’ve been treated and your partner hasn’t, reinfection during your next sexual encounter is very likely.
Where the Infection Takes Hold
The bacteria specifically target a type of cell called columnar epithelial cells, which line the cervix, urethra, rectum, and throat. Once the bacteria make contact with these cells, they enter them and essentially hijack the cell’s machinery. The bacteria exist in two forms: a small, hardy form that can survive outside of cells long enough to transfer between people during sex, and a larger form that replicates inside cells. After multiplying, the bacteria convert back to their infectious form and burst out to infect neighboring cells.
This is why chlamydia can infect several different body sites at the same time. Someone might have a genital infection and a rectal infection simultaneously if they’ve had both vaginal and anal sex with an infected partner. A throat infection is also possible after oral sex, though it tends to be less common and often clears on its own.
Why So Many People Spread It Unknowingly
The single biggest factor driving chlamydia transmission is the high rate of silent infections. Three out of four women and half of men with chlamydia experience no symptoms whatsoever. They feel completely fine, have no unusual discharge or discomfort, and have no reason to suspect they’re carrying a bacterial infection. Yet they are fully capable of passing it to a sexual partner.
When symptoms do appear, they typically show up one to three weeks after exposure. In women, this might look like unusual vaginal discharge, bleeding between periods, or a burning sensation when urinating. In men, symptoms usually involve discharge from the penis, burning during urination, or pain and swelling in one or both testicles. Rectal infections can cause discharge, pain, or bleeding from the rectum regardless of sex.
Because most infections are silent, routine screening is the only reliable way to catch chlamydia early. The CDC recommends annual screening for all sexually active women under 25 and for older women with risk factors like new or multiple partners.
Transmission During Childbirth
Chlamydia can also pass from a pregnant woman to her baby during vaginal delivery. As the baby moves through the birth canal, it can pick up the bacteria, which may lead to eye infections or pneumonia in the newborn. Both of these complications are serious but preventable with prenatal testing and treatment before delivery. Screening for chlamydia is a standard part of prenatal care for this reason.
How You Cannot Get Chlamydia
The bacterium that causes chlamydia cannot survive outside the human body. You cannot catch it from a toilet seat, a swimming pool, a hot tub, sharing towels, or touching a doorknob. Casual contact like hugging, kissing, or sharing food and drinks does not transmit the infection. The bacteria require direct contact with mucous membranes to establish an infection, which is why sexual contact is effectively the only route of person-to-person spread (aside from childbirth).
Who Is Most at Risk
Women are disproportionately affected. In 2024, women accounted for nearly 944,000 cases compared to about 564,000 in men, a rate of roughly 550 per 100,000 women versus 335 per 100,000 men. Part of this gap reflects higher screening rates in women, but biology also plays a role. The cervix is particularly susceptible to the bacteria, and younger women have a cervical anatomy that makes infection even easier.
Your risk increases with multiple sexual partners, a new sexual partner, inconsistent condom use, or a previous STI. Having had chlamydia before is itself a risk factor, both because it signals higher-risk sexual behavior and because it offers zero lasting protection against reinfection.
How Condoms Reduce Risk
Consistentcios and correct condom use reduces the risk of chlamydia by about 60%. That’s significant but notably lower than condom effectiveness against some other infections (condoms reduce gonorrhea risk by about 90%, for comparison). The gap exists because chlamydia can be present on genital skin or mucosal surfaces that a condom doesn’t fully cover. Still, condoms remain one of the most effective tools for reducing transmission, especially when combined with regular screening and prompt treatment of infections.
For people who have been treated for chlamydia, avoiding sex for at least seven days after completing treatment (and ensuring your partner has also been treated) is essential to prevent passing the infection back and forth.