Females get chlamydia through sexual contact with an infected partner. Vaginal, anal, and oral sex can all transmit the bacteria, and a condom-free encounter with someone who carries the infection is the primary route. Because roughly 75% of women with chlamydia have no symptoms, many people pass it along without knowing they’re infected, which makes understanding how transmission works especially important.
Sexual Contact Is the Primary Route
Chlamydia spreads through contact with infected sexual fluids or mucous membranes. Vaginal sex is the most common way females acquire the infection, but it’s not the only one. Anal sex can lead to a rectal chlamydia infection, and oral sex can transmit the bacteria to the throat. In some cases, the infection can also spread from one site to another on the same person, for example from the vagina to the rectum, without a separate sexual exposure.
The bacterium that causes chlamydia targets a specific type of cell lining the reproductive and urinary tracts. Once it makes contact, it attaches to those cells, enters them within hours, and begins multiplying rapidly inside. This is why transmission requires direct contact with infected tissue or fluids rather than casual contact like sharing a toilet seat or towel.
Lower-Risk Routes Still Carry Some Danger
Penetrative sex isn’t the only way the infection can spread. Sharing sex toys without washing them between uses or covering them with a fresh condom can transmit chlamydia. Fingering and other digital contact carry a lower risk, but it increases if there are any cuts or sores on the fingers, even tiny ones. These routes are less common than vaginal or anal intercourse, but they aren’t zero-risk.
Why Younger Women Face Higher Risk
Biology plays a role in susceptibility, particularly for younger women. The cervix has a transition zone where two types of tissue meet. In adolescents and young adults, this zone sits in a more exposed position on the outer part of the cervix. Chlamydia specifically targets the type of cells found in that zone, so its exposed location in younger women gives the bacteria easier access. As women move into their mid-twenties, this tissue gradually recedes to a more protected position inside the cervical canal.
This biological reality is one reason the CDC recommends annual chlamydia screening for all sexually active women under 25. For women 25 and older, screening is recommended if you have risk factors: a new sexual partner, more than one partner, a partner who has other partners, inconsistent condom use outside a mutually monogamous relationship, or a previous STI.
Most Infections Cause No Symptoms
Three out of four women with chlamydia never develop noticeable symptoms. This is what makes the infection so effective at spreading. You can carry it for weeks or months, transmit it to partners, and never realize anything is wrong. When symptoms do appear, they typically include unusual vaginal discharge, burning during urination, or bleeding between periods, but waiting for symptoms is an unreliable strategy for catching the infection.
After exposure, chlamydia takes about one to two weeks to reach levels a test can detect. A urine sample or vaginal swab taken one week after exposure will catch most infections, and testing at two weeks catches nearly all of them. If you’ve been treated for chlamydia, the CDC recommends retesting about three months later, since reinfection is common.
Transmission During Pregnancy and Birth
A pregnant woman with chlamydia can pass the infection to her baby during vaginal delivery. This can cause eye infections (conjunctivitis) or pneumonia in the newborn. Untreated chlamydia during pregnancy has also been linked to complications including preterm birth.
Because of these risks, all pregnant women under 25 are screened for chlamydia, and pregnant women 25 and older are screened if they have risk factors. Women who test positive and receive treatment are retested four weeks later to confirm the infection has cleared, then again during the third trimester to catch any reinfection before delivery.
How Reinfection Happens
Having chlamydia once does not protect you from getting it again. Reinfection is one of the most common patterns with this STI, and it typically happens for a straightforward reason: a treated partner returns to sex with an untreated partner. Both people in a sexual relationship need to complete treatment, and you should wait until treatment is finished before having sex again. The roughly 944,000 chlamydia cases reported among U.S. women in 2024 alone reflect how easily and frequently the infection circulates, often cycling between the same partners.
Using condoms consistently and correctly during vaginal, anal, and oral sex is the most effective way to reduce your risk. Getting screened on the recommended schedule is the most reliable way to catch an infection you can’t feel, treat it before it causes damage to your reproductive tract, and avoid passing it to someone else.