Chlamydia spreads through vaginal, anal, or oral sex without a condom with someone who has the infection. It does not require ejaculation to pass from one person to another. What makes chlamydia especially easy to catch is that most people who have it don’t know it: roughly 75% of women and 50% of men with chlamydia show no symptoms at all, meaning they can pass the bacteria along without realizing they’re infected.
Sexual Contact Is the Primary Route
Chlamydia is caused by the bacterium Chlamydia trachomatis, and it travels through sexual fluids and contact with infected mucous membranes. Any unprotected vaginal, anal, or oral sex with an infected partner can transmit it. Penetration doesn’t need to be deep, and ejaculation doesn’t need to happen. Pre-ejaculate fluid and direct contact between mucous membranes (the moist lining of the genitals, rectum, or throat) are enough for the bacteria to transfer.
Vaginal and anal sex carry the highest risk. Oral sex can also spread chlamydia, though it’s less commonly discussed. Giving oral sex to a partner who has a genital infection can lead to a throat (pharyngeal) infection, and receiving oral sex from someone with a throat infection can lead to a genital one. Pharyngeal chlamydia rarely causes noticeable symptoms, which makes it easy to miss entirely.
Why So Many People Spread It Unknowingly
The single biggest factor driving chlamydia transmission is that most infections produce no symptoms. Three out of four women and half of men with chlamydia feel completely fine. They have no pain, no unusual discharge, nothing that would prompt them to get tested or avoid sex. This is why chlamydia is sometimes called a “silent” infection, and it’s the reason routine screening matters so much for sexually active people, particularly those under 25 or with new partners.
When symptoms do appear, they typically show up one to three weeks after exposure. In women, that might look like unusual vaginal discharge, burning during urination, or bleeding between periods. In men, it’s usually a discharge from the penis, burning when urinating, or pain in the testicles. Rectal infections can cause discharge, pain, or bleeding. But again, the absence of any of these symptoms does not mean you’re in the clear.
Reinfection From an Untreated Partner
One of the most common ways people get chlamydia a second time is by having sex again with a partner who was never treated. This “ping-pong” pattern happens when one person gets treated and the other doesn’t, so the bacteria passes right back. According to the California Department of Public Health, the most common reason for repeat chlamydia infections is sex with a partner who still carries the infection. Both partners need to be treated at the same time for treatment to stick. If your partner isn’t treated, your own course of antibiotics won’t protect you for long.
Mother-to-Child Transmission
Chlamydia can also pass from a pregnant person to their baby during vaginal delivery. As the baby moves through the birth canal, the bacteria can infect the newborn’s eyes or lungs. This can lead to eye infections (neonatal conjunctivitis) or pneumonia in the first weeks of life. Prenatal screening catches most cases before delivery, and treatment during pregnancy prevents transmission.
Ways You Cannot Get Chlamydia
Chlamydia bacteria cannot survive long outside the human body. You will not get chlamydia from a toilet seat, a swimming pool, sharing food, hugging, or casual contact. The bacteria need the warm, moist environment of mucous membranes to survive, and exposure to air kills them quickly. You also can’t get it from sharing towels or clothing. These myths persist, but the transmission route is essentially limited to sexual contact and childbirth.
How Soon It Shows Up on a Test
If you’ve had unprotected sex and want to know whether you were exposed, timing matters. Chlamydia testing (typically a urine sample or swab) can detect the infection as early as one week after exposure in most cases. Waiting two weeks catches nearly all infections. Testing too early, within the first few days, can produce a false negative because the bacteria haven’t multiplied enough to be detected yet.
If you develop symptoms within one to three weeks of a sexual encounter, that’s a strong signal to get tested. But since most infections are symptomless, getting tested based on risk factors alone (new partner, unprotected sex, partner diagnosed with an STI) is the more reliable approach.
How Common Chlamydia Is
Chlamydia remains the most frequently reported bacterial sexually transmitted infection in the United States. In 2024, over 2.2 million combined cases of chlamydia, gonorrhea, and syphilis were reported to the CDC, with chlamydia making up the largest share. Cases did decline 8% from 2023 to 2024, but the numbers are still enormous. Young adults, particularly those between 15 and 24, account for a disproportionate number of cases.
Reducing Your Risk
Condoms used consistently and correctly during vaginal, anal, and oral sex significantly reduce the risk of chlamydia transmission. They don’t eliminate it entirely, since contact can still occur around the edges of the condom, but they are the most effective barrier method available.
Getting tested regularly, especially with new partners, catches infections before they have a chance to spread or cause complications. If you test positive, making sure your recent sexual partners are also tested and treated prevents reinfection and breaks the chain of transmission. Chlamydia is curable with a short course of antibiotics, but it can cause serious reproductive harm if left untreated, including pelvic inflammatory disease and fertility problems in women.