Chlamydia spreads through sexual contact, specifically vaginal, anal, or oral sex without a condom with someone who has the infection. It can also pass from a mother to her baby during childbirth. You cannot get chlamydia from toilet seats, sharing towels, or casual contact.
How the Bacteria Spreads During Sex
Chlamydia is caused by the bacterium Chlamydia trachomatis, which lives exclusively in the mucous membranes of the body: the lining of the vagina, penis, rectum, and throat. It cannot survive in open air or on surfaces like toilet seats, doorknobs, or shared clothing. This is why sexual contact is essentially the only way the infection moves between people.
During vaginal or anal sex, the bacteria transfer from one person’s infected tissue to another’s. Vaginal sex is the most common route. Anal sex, specifically being the receptive partner, can lead to a rectal infection. The bacteria can also spread from an infected vagina to the rectum without anal sex occurring. Oral sex can transmit chlamydia to the throat, though throat infections are less commonly diagnosed and often produce no symptoms at all.
The infection takes hold at the specific site of contact. That means you can have chlamydia in your throat, your genitals, or your rectum independently. Testing only the genital area can miss infections at other sites entirely.
Why So Many People Spread It Without Knowing
The single biggest reason chlamydia spreads so effectively is that most people who have it feel completely fine. At least 70% of women and 50% of men with a genital chlamydia infection have no symptoms at all at the time of diagnosis. They don’t know they’re infected, so they don’t seek treatment, and transmission continues uninterrupted.
When symptoms do appear, they typically show up one to three weeks after exposure. Men may notice discharge from the penis or a burning sensation while urinating. Women may experience unusual vaginal discharge, bleeding between periods, or pelvic pain. But because the majority of cases are silent, regular screening is the only reliable way to catch it.
What Happens Inside Your Body
The cells lining your genital tract, rectum, or throat don’t naturally absorb bacteria the way immune cells do. Chlamydia has to force its way in. It does this by injecting specialized proteins into the surface of your cells. These proteins hijack your cell’s internal scaffolding, causing tiny finger-like projections on the cell surface to grow larger and eventually wrap around the bacterium, pulling it inside.
Once inside, the bacterium creates a protective bubble called an inclusion. Think of it as a command center. From within this bubble, chlamydia manipulates the host cell’s processes to feed itself, replicate, and avoid being detected by your immune system. Eventually the cell releases new bacteria, which go on to infect neighboring cells. This cycle of silent replication is why untreated chlamydia can cause serious damage over time, particularly to a woman’s reproductive system, leading to pelvic inflammatory disease, scarring of the fallopian tubes, and infertility.
Transmission During Childbirth
A pregnant person with an untreated chlamydia infection can pass it to their baby during vaginal delivery. As the infant moves through the birth canal, it comes into direct contact with infected tissue. The most common result is neonatal conjunctivitis, a serious eye infection that can appear in the first few weeks of life. Chlamydia can also cause pneumonia in newborns. This is why routine chlamydia screening is standard during prenatal care.
Who Faces the Highest Risk
Anyone who is sexually active can get chlamydia, but certain factors raise the likelihood. Having a new sexual partner or multiple partners increases exposure. Not using condoms consistently is a major factor. Younger women are disproportionately affected, in part because they are screened more often but also because of higher rates of partner change in younger age groups.
Research has also identified some less obvious risk factors. Having a concurrent HPV infection was independently associated with a higher chance of acquiring chlamydia, likely through a combination of immune system effects and overlapping sexual networks. Smoking and substance use beyond alcohol and marijuana were also linked to increased risk, possibly because they suppress local immune defenses or correlate with higher-risk sexual behavior.
How Well Condoms Protect You
Condoms reduce the risk of chlamydia significantly, but the degree of protection depends on how consistently and correctly they’re used. One study of urban adolescents found that correct and consistent condom use reduced the risk of chlamydia by about 60%. A separate study in London found a 90% reduction among women who used condoms consistently with their regular partners. The gap between those numbers comes down to real-world use: condoms work best when they’re used every single time, from start to finish.
Internal (female) condoms and dental dams offer barrier protection for receptive vaginal or anal sex and oral sex, respectively, though they are used far less frequently and studied less extensively.
How Soon It Shows Up on a Test
If you’ve been exposed, a test can reliably detect chlamydia about one week after exposure in most cases. Waiting two weeks catches nearly all infections. Testing too early, within the first few days, risks a false negative because the bacteria haven’t replicated enough to be picked up. Testing is done through a urine sample or a swab of the vagina, rectum, or throat, depending on the site of potential exposure.
Because symptoms take one to three weeks to appear when they appear at all, and because most infections are completely silent, timing your test correctly matters more than waiting for symptoms. 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 a reliable result.
Ways You Cannot Get Chlamydia
Chlamydia bacteria die almost immediately outside the body’s warm, moist mucous membranes. You cannot get chlamydia from a toilet seat, a swimming pool, sharing food or drinks, hugging, holding hands, or breathing the same air as someone who is infected. The bacteria require direct mucous membrane contact to transfer, which in practical terms means sexual activity or passage through the birth canal.