Chlamydia spreads through sexual contact with an infected person. The bacteria responsible, called Chlamydia trachomatis, passes from one person to another when mucous membranes come into direct contact during vaginal, anal, or oral sex. You don’t need to have penetrative sex to transmit it, and most people who have it don’t know they’re infected.
How the Bacteria Spreads During Sex
Chlamydia trachomatis lives in the warm, moist tissue that lines the genitals, rectum, and throat. During sex, the bacteria transfer from an infected person’s mucous membranes to a partner’s through direct contact with infected fluids or tissue. This can happen during vaginal sex, anal sex, or oral sex. Penetrative sex carries the highest risk, but any genital-to-genital or mouth-to-genital contact can be enough.
The infection takes hold wherever the bacteria land. Vaginal sex can lead to infection of the cervix or urethra. Anal sex can cause a rectal infection. Oral sex can result in a throat infection, though this is less common. You can also spread the bacteria from one body site to another on your own, for example transferring it from the genitals to the eyes by touching.
Ejaculation is not required for transmission. Simply having unprotected contact with infected tissue is sufficient. This is one reason chlamydia spreads so efficiently: people often underestimate the risk of specific sexual acts because they assume penetration or ejaculation has to occur.
Why Most People Don’t Realize They Have It
The single biggest factor driving chlamydia transmission is that the infection is usually silent. About 75% of women and 50% of men with chlamydia have no symptoms at all. A person can carry and spread the bacteria 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 look like unusual vaginal discharge, burning during urination, or bleeding between periods. In men, symptoms can include discharge from the penis, burning with urination, or testicular pain. Rectal infections may cause discharge, pain, or bleeding, but often produce no symptoms at all. Because these signs are mild or absent, many people unknowingly pass the infection to new partners.
Ways You Cannot Get Chlamydia
Chlamydia cannot survive outside the human body for more than a very short time. You cannot get it from a toilet seat, a swimming pool, sharing towels, hugging, kissing, or sharing food. The bacteria require the specific conditions of human mucous membranes to survive and replicate. Casual, non-sexual contact poses no risk.
Transmission During Childbirth
The one non-sexual route of transmission is from mother to baby during vaginal delivery. A pregnant person with an active chlamydia infection can pass the bacteria to the newborn as the baby moves through the birth canal. This can cause eye infections or pneumonia in the infant. Routine prenatal screening catches most cases before delivery, and treatment during pregnancy prevents transmission.
How Quickly It Becomes Detectable
After exposure, chlamydia typically becomes detectable on a test within about one week, though waiting two weeks catches nearly all infections. The most reliable test is a nucleic acid amplification test (NAAT), which detects the bacteria’s genetic material from a urine sample or a swab of the vagina, rectum, or throat. It’s simple, fast, and highly accurate.
If you’ve had unprotected sex with a new partner or a partner whose status you don’t know, testing at the two-week mark gives you the most reliable result. Because symptoms are so unreliable as a signal, routine screening is the only dependable way to catch an infection early.
Getting Chlamydia More Than Once
Having chlamydia once does not protect you from getting it again. Your body does not build lasting immunity to the bacteria, and reinfection is common. Studies have found that 18% to 34% of women who tested positive and completed treatment tested positive again within three to twelve months. In most of those cases, genomic analysis showed the follow-up infections involved new strains of the bacteria, meaning they were genuine reinfections from sexual contact rather than treatment failures.
Reinfection happens most often when a person is treated but their sexual partner is not. The partner continues carrying the bacteria and passes it right back. This is why treatment guidelines recommend that all recent sexual partners be notified and treated at the same time. Without that step, the cycle of transmission simply restarts.
Who Is Most at Risk
Chlamydia is the most commonly reported bacterial sexually transmitted infection in the United States, with more than 2.2 million combined cases of chlamydia, gonorrhea, and syphilis reported in 2024. Sexually active people under 25 have the highest infection rates, partly because of higher rates of partner change and lower rates of consistent condom use. But chlamydia affects people of all ages and backgrounds.
Your risk increases with each unprotected sexual encounter with a new or untested partner. Using condoms consistently and correctly during vaginal and anal sex significantly reduces transmission. For oral sex, dental dams or condoms offer protection. Regular screening, particularly if you have new or multiple partners, remains the most effective way to catch infections before they cause complications or spread further.