Chlamydia spreads through sexual contact with an infected person, primarily during vaginal, anal, or oral sex without a condom. It’s the most commonly reported bacterial sexually transmitted infection in the United States, with chlamydia contributing to more than 2.2 million combined STI cases reported in 2024. What makes it especially easy to pass along is that most people who have it don’t know: roughly 75% of infected women and 50% of infected men never develop symptoms.
The Main Ways Chlamydia Spreads
The bacterium that causes chlamydia, called Chlamydia trachomatis, passes from one person to another through contact with infected mucous membranes. In practical terms, that means any unprotected sexual contact where these tissues touch or exchange fluids can transmit the infection. Vaginal sex is the most common route, but anal and oral sex also carry risk.
Rectal infections can happen two ways: through receptive anal sex, or by the bacteria spreading from an infected vagina to the rectum on its own. Throat infections are possible through oral sex, though they tend to be less common and often produce no symptoms at all. Because the infection can live in the throat, rectum, or genitals independently, you can be infected at one site and test negative at another. This is why testing at each relevant body site matters, not just a urine test.
Why So Many Cases Go Unnoticed
The single biggest factor driving chlamydia transmission is that the infection is usually silent. Three out of four women and half of men with chlamydia experience no symptoms whatsoever. They feel fine, have no unusual discharge or pain, and have no reason to suspect anything is wrong. Meanwhile, they can pass the bacteria to every sexual partner.
When symptoms do appear, they typically show up one to three weeks after exposure. Women may notice unusual vaginal discharge or burning during urination. Men might see discharge from the penis or feel testicular discomfort. Rectal infections can cause discharge, bleeding, or pain, but often cause nothing at all. Because these symptoms are mild and can mimic other conditions, many people dismiss them even when they do occur.
How the Bacteria Actually Infects You
Chlamydia trachomatis has a two-stage life cycle that makes it particularly effective at establishing infection. The bacteria exist in a dormant, hardy form that can survive outside cells and attach to the delicate lining of the cervix, urethra, rectum, or throat. Once attached, they get pulled inside your cells, where they switch into an active, reproducing form. They multiply inside a protective bubble within the cell, then burst out as new dormant particles ready to infect more cells or spread to a partner.
This intracellular lifestyle is part of why chlamydia is so sneaky. The bacteria essentially hide inside your own cells, which helps them evade your immune system and explains why infections can persist for months without triggering obvious symptoms.
Transmission During Childbirth
Chlamydia can also pass from mother to baby during vaginal delivery. About 50% of infants born vaginally to infected mothers pick up the bacteria during birth. These newborns can develop eye inflammation with redness, swelling, and discharge, sometimes along with pneumonia that causes coughing and rapid breathing. This is one reason prenatal STI screening is standard practice.
You Can’t Get It From Toilet Seats or Pools
Chlamydia trachomatis can technically survive on surfaces for two to three hours under humid conditions, but that doesn’t translate into real-world risk. The bacteria need direct contact with mucous membrane tissue to establish infection. You cannot get chlamydia from a toilet seat, swimming pool, hot tub, shared towel, or casual contact like hugging or shaking hands. It requires the kind of direct, intimate mucosal contact that happens during sex.
How Well Condoms Protect You
Consistent condom use significantly reduces your risk. A study published in the American Journal of Epidemiology found that among people with confirmed exposure to an infected partner, using condoms every time was associated with a 58% reduction in chlamydia and gonorrhea infections. That’s a meaningful level of protection, though not perfect, since condoms don’t cover all genital skin and aren’t always used for oral sex.
The protective effect drops when researchers look at broader populations where partner infection status is unknown, likely because some of those “exposed” people weren’t actually having sex with infected partners to begin with, which dilutes the numbers. The takeaway: condoms work well against chlamydia, but they’re most effective as part of a broader approach that includes regular testing.
Testing Windows and Detection
If you think you’ve been exposed, testing too early can give a false negative. The bacteria need time to multiply enough to be detectable. Most infections show up on a test after one week, and waiting two weeks catches nearly all cases. This is the window between exposure and reliable detection, so timing your test matters.
Testing itself is straightforward: a urine sample or a swab of the vagina, rectum, or throat depending on the type of sexual contact you’ve had.
Reinfection Is Common
Having chlamydia once doesn’t protect you from getting it again. In fact, reinfection rates are high. The CDC notes a high prevalence of repeat chlamydia infections among people treated in the preceding months. The vast majority of these aren’t treatment failures. They happen because the infected person’s partner didn’t get treated, or because the person started having sex with a new partner who was also infected.
After treatment, you should avoid sex for seven days to prevent passing the bacteria to anyone else. Your partners need treatment too, even if they have no symptoms, or reinfection is almost inevitable. Retesting about three months after treatment is recommended regardless of whether you believe your partners were treated, because reinfection is that common and chlamydia left untreated can lead to serious complications including pelvic inflammatory disease and fertility problems.
Who Is Most at Risk
Sexually active people under 25 have the highest rates of chlamydia, partly because of higher rates of new and multiple partners and lower rates of consistent condom use. Men who have sex with men face additional risk for rectal and throat infections that standard urine tests miss entirely. The CDC recommends at least yearly screening for rectal chlamydia in men who have had receptive anal sex, with more frequent screening every three to six months for those with multiple or anonymous partners.
Anyone who is sexually active with new or multiple partners benefits from regular screening. Because the infection is so often asymptomatic, routine testing is the only reliable way to catch it before it spreads or causes damage.