Chlamydia spreads through vaginal, anal, or oral sex without a condom with someone who has the infection. It is the most commonly reported bacterial sexually transmitted infection in the United States, contributing to more than 2.2 million combined STI cases reported in 2024 alongside gonorrhea and syphilis. What makes chlamydia especially easy to catch is that most people who have it don’t know it: roughly 75% of infected women and 50% of infected men never develop noticeable symptoms, yet they can still pass the bacteria to a partner.
Sexual Contact Is the Primary Route
Chlamydia is caused by the bacterium Chlamydia trachomatis, which infects the mucous membranes of the genitals, rectum, and throat. Any unprotected sexual contact that involves these areas can transmit it. That includes vaginal intercourse, anal intercourse (both giving and receiving), and oral sex. You don’t need to experience ejaculation for transmission to occur. Direct contact between mucous membranes is enough.
Rectal infections deserve special mention because they can happen two ways. Receptive anal sex is the obvious route, but the bacteria can also spread from an infected vagina to the rectum on its own, without anal sex being involved. This means someone who has never had anal sex can still develop a rectal chlamydia infection.
Why So Many Cases Go Unnoticed
The high rate of symptomless infection is the main reason chlamydia spreads so effectively. Three out of four women and half of men with the infection feel perfectly fine. Without symptoms prompting a doctor visit, the infection can persist for weeks or months, giving it plenty of opportunity to pass to 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, the most common signs are a clear or cloudy discharge from the penis and a burning sensation when urinating. Rectal infections can cause discharge, pain, or bleeding, while throat infections are usually silent.
Because so many infections are invisible, routine screening is the most reliable way to catch them. Testing becomes accurate about one week after exposure in most cases and catches nearly all infections by the two-week mark. A simple urine sample or swab is all that’s needed.
Transmission During Childbirth
A pregnant person with an untreated chlamydia infection can pass the bacteria to their baby during vaginal delivery. The newborn’s exposure to infected tissue in the birth canal puts the baby at risk for chlamydial conjunctivitis, an eye infection that causes redness, swelling, and discharge, and chlamydial pneumonia, a lung infection that can develop in the first few months of life. This is why screening during pregnancy is standard practice, with retesting recommended in the third trimester to catch any infections acquired later in the pregnancy and prevent complications for both parent and child.
Spreading It to Your Own Eyes
It’s possible to transfer chlamydia from your genitals to your eyes by touching infected secretions and then touching your eye. This hand-to-eye spread of infected genital secretions is the usual route for adult chlamydial conjunctivitis, which causes persistent redness, irritation, and discharge that can last weeks if untreated. It’s not common, but it’s a reminder that hand hygiene matters during and after sexual contact.
How Chlamydia Infects Your Cells
Once the bacteria reaches a mucous membrane, it has a remarkably efficient method of getting inside your cells. The bacterium exists in two forms: a small, tough “seed” form designed for traveling between people, and a larger, fragile form that replicates once safely inside a cell. The seed form is tiny, about 250 nanometers across, with a compacted, hardy outer shell that lets it survive briefly outside a cell.
When this seed form contacts a cell lining your cervix, urethra, rectum, or throat, it injects proteins that hijack the cell’s internal scaffolding, essentially tricking the cell into pulling the bacterium inside. Once in, the bacterium creates a protective bubble called an inclusion, where it transforms into its replicating form and multiplies. The full cycle takes about 48 to 96 hours, after which new seed forms burst out to infect neighboring cells or wait to be transmitted to a new host. This cycle of invasion, replication, and release is why untreated chlamydia can cause progressive tissue damage over time.
Ways You Cannot Get Chlamydia
Chlamydia bacteria cannot survive long outside the human body. You cannot get chlamydia from toilet seats, swimming pools, hot tubs, shared towels, or doorknobs. The bacteria need the warm, moist environment of human mucous membranes to stay alive, and hard surfaces simply don’t provide that. Casual contact like hugging, shaking hands, sharing food, or sitting next to someone poses zero risk.
You also can’t catch it from a pet. While some animals carry their own species of Chlamydia, the strain that infects humans sexually (Chlamydia trachomatis) spreads only between people through the routes described above.
Who Faces the Highest Risk
Anyone who is sexually active can get chlamydia, but certain factors increase the odds. Having a new sexual partner, having multiple partners, or not using condoms consistently all raise your risk. Younger people, particularly women under 25, have higher infection rates partly because the cells lining the cervix are more susceptible to the bacteria at younger ages. A previous chlamydia infection doesn’t protect you from getting it again. Reinfection is common and carries the same risks as the first time around, including potential damage to the reproductive tract if left untreated.
Consistent condom use significantly reduces transmission risk but doesn’t eliminate it entirely, since skin-to-skin contact around areas not covered by the condom can still allow the bacteria to spread. Regular screening, especially for sexually active women under 25 and anyone with new or multiple partners, remains the most effective way to catch infections early and break the chain of transmission.