Chlamydia spreads through sexual contact with an infected person. The bacteria pass from one person to another during vaginal, anal, or oral sex, making it the most commonly reported bacterial sexually transmitted infection in the United States, with over 1.5 million cases reported in 2024 alone. You do not need to have symptoms to spread it, and most people who are infected don’t know they are.
How the Bacteria Enters Your Body
Chlamydia is caused by the bacterium Chlamydia trachomatis, which targets the thin, moist tissue lining your genital tract, rectum, and throat. When this tissue comes into direct contact with an infected partner’s genital secretions during sex, the bacteria attach to the surface cells and enter them. Within about two hours, they begin multiplying rapidly inside those cells.
This is why chlamydia can only be transmitted through close physical contact with mucous membranes. The bacterium does not survive outside the human body, which means you cannot get chlamydia from a toilet seat, swimming pool, shared towel, or doorknob. It requires direct, person-to-person sexual contact.
Vaginal, Anal, and Oral Sex All Carry Risk
Vaginal sex is the most common route of transmission. During unprotected vaginal intercourse, the bacteria can infect the cervix in women and the urethra in men. Anal sex also carries significant risk, and rectal chlamydia infections are common, particularly among men who have sex with men. These infections can occur regardless of which partner is penetrating.
Oral sex is a less efficient route, but it can still transmit chlamydia. A throat infection with chlamydia is possible after performing oral sex on an infected partner, and someone with chlamydia in their throat can potentially pass it to a partner’s genitals. Throat infections often cause no symptoms at all, and the CDC notes they may be less harmful than genital or rectal infections, though they still contribute to ongoing spread.
Importantly, ejaculation does not need to occur for transmission. Simple contact between infected mucous membranes is enough. Sharing sex toys without cleaning them between uses can also transfer the bacteria from one person to another.
Why Most People Don’t Know They’re Infected
The single biggest factor driving chlamydia transmission is that most carriers have no idea they have it. About 75% of women and 50% of men with chlamydia experience no symptoms whatsoever. This means a person can carry and spread the infection for weeks or months without any sign that something is wrong.
When symptoms do appear, they typically show up one to three weeks after exposure. In women, this might include unusual vaginal discharge, burning during urination, or bleeding between periods. In men, symptoms can include discharge from the penis, burning with urination, or pain in one or both testicles. Rectal infections may cause discharge, pain, or bleeding, but often produce no symptoms at all.
Because the infection is so frequently silent, routine screening is the only reliable way to catch it. If you’ve been exposed, testing is accurate as early as one week after contact, and waiting two weeks catches nearly all infections.
Transmission During Childbirth
Chlamydia can also pass from mother to baby during vaginal delivery. As the infant moves through the birth canal, the bacteria can infect the baby’s eyes, causing conjunctivitis (an eye infection), or the lungs, leading to pneumonia in the first few months of life. This is why prenatal screening for chlamydia is standard practice, and treating the infection during pregnancy prevents these complications in newborns.
Reinfection Is Common
Having chlamydia once does not protect you from getting it again. Your body does not build lasting immunity to the bacteria after clearing an infection. In studies of adolescents and young adults who were retested within 12 months of treatment, 22% tested positive for chlamydia again.
Reinfection usually happens for straightforward reasons: a treated person resumes sex with an untreated partner, or they have a new partner who is unknowingly infected. This is why both partners need to be treated at the same time, and why retesting a few months after treatment is recommended. Each new infection carries the same risks of complications, including pelvic inflammatory disease in women and potential fertility problems for both sexes.
Who Gets Chlamydia Most Often
Anyone who is sexually active can contract chlamydia, but certain groups face higher rates. Women are diagnosed nearly twice as often as men, partly because screening programs target women more aggressively (especially during gynecological visits and prenatal care), and partly because the anatomy of the cervix makes it particularly susceptible to infection. In 2024, the reported rate among women was about 550 cases per 100,000 people, compared to 335 per 100,000 among men.
Young people between 15 and 24 account for a disproportionate share of cases. Having multiple sexual partners, inconsistent condom use, and a previous STI all increase the likelihood of infection. The gap between male and female case counts also reflects the fact that half of infected men have no symptoms and may never get tested, meaning the true number of male infections is likely higher than reported figures suggest.
What Reduces Your Risk
Condoms, when used consistently and correctly during vaginal, anal, and oral sex, significantly reduce the chance of transmission. They don’t eliminate risk entirely because chlamydia can infect any mucosal tissue that makes contact, but they remain the most effective barrier method available.
Regular screening is equally important, especially if you’re under 25, have a new partner, or have multiple partners. Because chlamydia so rarely announces itself with symptoms, testing is the only way to know your status. If you do test positive, treatment with antibiotics clears the infection, but you’ll need to abstain from sex for seven days after completing treatment and make sure your partner is treated as well to avoid passing it back and forth.