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 reported in 2024 alone. You can get it through vaginal, anal, or oral sex, and most people who carry it have no idea they’re infected.
Sexual Contact Is the Primary Route
Chlamydia passes from one person to another when mucous membranes come into contact with the bacteria. This happens during vaginal sex, anal sex, and oral sex. The bacteria infects the cells lining the cervix, urethra, rectum, and throat. You don’t need to have penetrative sex to be exposed; any genital-to-genital or mouth-to-genital contact with an infected area can transmit it.
The infection can also establish itself in more than one location at the same time. You could have chlamydia in your throat and your genitals simultaneously, for example, without knowing about either. Throat infections from oral sex may produce fewer or no symptoms, but they can still spread the bacteria to a partner’s genitals during future oral contact.
Ejaculation is not required for transmission. The bacteria lives on the mucosal surfaces themselves, so any unprotected contact with an infected site carries risk.
Most Infected People Show No Symptoms
One of the reasons chlamydia spreads so easily is that the majority of carriers don’t look or feel sick. About 75% of women and 50% of men with chlamydia have no symptoms at all. When symptoms do appear, they typically take several weeks after exposure to develop.
This means your sexual partner could have chlamydia and genuinely not know it. There’s no way to tell by looking at someone whether they’re carrying the infection. The only reliable way to know is through testing.
You Can Get It in the Throat or Rectum
Chlamydia isn’t limited to the genitals. Giving oral sex to a partner who has a genital or rectal infection can lead to a throat infection. Receiving anal sex from an infected partner can cause rectal chlamydia. These infections often produce no symptoms, which means they can go undetected for months.
Rectal chlamydia sometimes causes discharge, pain, or bleeding, but many people experience nothing at all. Throat chlamydia is even less likely to cause noticeable symptoms. Both can still be passed to other partners.
Transmission During Childbirth
A pregnant person with untreated chlamydia can pass the bacteria to their baby during vaginal delivery. The newborn picks up the infection from the mother’s cervix as it passes through the birth canal. The most common result is conjunctivitis, an eye infection that typically appears 5 to 12 days after birth. In some cases, the baby can develop pneumonia between 1 and 3 months of age.
This is why screening during pregnancy is standard practice. All pregnant people under 25, and those over 25 with risk factors, are recommended to be tested.
How It Cannot Spread
Chlamydia bacteria cannot survive outside the body’s mucous membranes. You cannot get chlamydia from a toilet seat, a swimming pool, sharing towels, hugging, kissing, or shaking hands. The bacteria dies quickly on hard surfaces and in open air. Casual, non-sexual contact poses no risk.
What Happens Inside the Body
When chlamydia bacteria reach a mucosal surface, they attach to the epithelial cells lining that tissue. The bacteria then essentially trick those cells into absorbing them. Once inside, the bacteria create a protected pocket called an inclusion, which shields them from the immune system. Inside this pocket, they multiply. Eventually the host cell either ruptures or the bacteria push their way out as an intact bundle, releasing new infectious particles that go on to infect neighboring cells.
This cycle can continue silently for months. Left untreated, the ongoing infection and inflammation can damage reproductive tissue over time, potentially leading to pelvic inflammatory disease, chronic pain, or fertility problems.
Reinfection Is Common
Having chlamydia once does not make you immune. If you’re treated but your partner isn’t, you can be reinfected the next time you have sex with them. This is why retesting about 3 months after treatment is recommended. Both partners need to complete treatment before resuming sexual contact.
Who Should Get Tested
Because most infections are silent, routine screening is the only way to catch chlamydia early. Current guidelines recommend annual testing for all sexually active women under 25, and for women 25 and older who have new or multiple partners, a partner with other partners, or inconsistent condom use. Men who have sex with men should be tested at least once a year at all sites of contact, and every 3 to 6 months if they’re on PrEP, have HIV, or have multiple partners.
Transgender and gender diverse people should follow screening recommendations based on their anatomy. Anyone with a cervix who is under 25 and sexually active falls under the same annual screening recommendation regardless of gender identity.
Testing is simple, usually involving a urine sample or a swab, and chlamydia is curable with antibiotics. The challenge is catching it, since the infection so rarely announces itself.