Chlamydia is transmitted through vaginal, anal, or oral sex with someone who carries Chlamydia trachomatis, because the bacterium passes in infected genital, rectal, or throat fluids. Penetration or ejaculation isn't required for contact to happen. A pregnant person can also pass it to their baby during childbirth CDC, About Chlamydia.
| Item | Reported cases |
|---|---|
| Chlamydia | 1,648,568 |
| Gonorrhea | 601,319 |
| Syphilis (P&S) | 53,007 |
How chlamydia spreads, route by route
Chlamydia is a bacterial infection of mucous membranes — the moist tissue lining the genitals, rectum, throat, and eyes. The organism doesn't survive long outside the body, so transmission happens when those surfaces make direct contact during sex and infected fluid moves from one person to another. It's the most commonly reported bacterial STI in the US, with roughly 1.65 million reported cases in the most recent surveillance year CDC AtlasPlus, 2023. Most people who carry it feel nothing, so they pass it on without any symptom to warn them.
Vaginal sex
Vaginal intercourse is the most common route. The bacterium infects the cervix in women and the urethra in men, and it travels in either direction as an infected partner's fluids reach the other partner's mucous membranes. Because the cervix often shows no irritation, a woman can carry it for months without knowing.
Anal sex
Receptive anal sex can seed a rectal infection, which is frequently silent or causes only vague discomfort, discharge, or pain. Rectal chlamydia matters clinically because it's commonly missed when testing only urine. It needs a rectal swab to detect, and it responds less well to older single-dose treatment, part of why guidelines shifted regimens (more on that under azithromycin vs doxycycline for chlamydia).
Oral sex
Oral sex can transmit chlamydia to the throat (pharyngeal infection) or, less often, from an infected throat to a partner's genitals. Throat infections rarely cause symptoms and are easy to overlook, so screening men who have sex with men is recommended at every site of exposure — urine, rectum, and throat.
Mother to baby during childbirth
A pregnant person with untreated chlamydia can pass it to their newborn as the baby moves through the birth canal. In infants, chlamydia is a leading cause of early conjunctivitis (eye inflammation) and pneumonia (a lung infection in the first weeks to months of life). Screening is built into routine prenatal care because catching and treating it before delivery prevents these outcomes.
How chlamydia is NOT spread
This is where worry usually runs ahead of biology. Chlamydia trachomatis can't survive on dry, everyday surfaces, so the things people fear most don't transmit it:
- Toilet seats, sinks, or other bathroom surfaces — the bacterium dies quickly outside the body and can't infect you from a seat.
- Towels, bedsheets, or shared clothing.
- Swimming pools, hot tubs, or saunas.
- Casual contact — hugging, holding hands, sharing a meal, or sitting next to someone.
- Saliva alone (a kiss without sexual contact); chlamydia isn't spread by ordinary kissing.
- Sharing cups, utensils, or drinks.
If you've already been treated, you're not immune. You can catch it again from an untreated or newly infected partner. Repeat infection is common, which is why retesting exists; see chlamydia reinfection.
Who's at higher risk
Risk tracks closely with age, sexual network, and screening gaps rather than anything about a person's character. The groups carrying the highest burden:
- Sexually active young women — the USPSTF recommends routine screening for all sexually active women 24 and under because their cervical tissue is more susceptible and the complication burden falls heavily on them USPSTF, 2021.
- Men who have sex with men (MSM), who carry a disproportionate share of rectal and pharyngeal infections and of invasive strains.
- Anyone with new or multiple partners, or a partner who has other partners.
- People who don't use condoms consistently, especially with a partner of unknown status.
- Geographically, rates run far above the national average in some areas. Washington DC, Louisiana, and Mississippi report more than double the US rate, reflecting differences in access to testing and care.
Screening guidance is deliberately asymmetric: there's strong evidence to screen young women but insufficient evidence to recommend routine screening of men, because the serious downstream complications — pelvic inflammatory disease, tubal scarring, infertility — concentrate in women. CDC still recommends at least annual screening for sexually active MSM at all exposure sites, and more often for those at higher risk.
Reducing your risk
Prevention works best as layers, not a single act. Because most infections are silent, the tools that move the numbers are routine testing and treating partners, not just condoms.
- Use condoms correctly and every time — they meaningfully lower risk for all three sexual routes, though they don't cover every area of skin contact.
- Get screened on schedule, even with no symptoms; a long-term, mutually monogamous relationship in which both partners have tested negative is its own form of protection.
- Treat partners. In a landmark randomized trial, giving patients medication to pass to partners (expedited partner therapy) reduced persistent or repeat infection Golden et al., NEJM.
- For eligible people, DoxyPEP is an option — a single dose of doxycycline taken within 72 hours after sex reduced chlamydia by more than 70% in trials, and CDC offers it to MSM and transgender women who've had a bacterial STI in the prior year CDC DoxyPEP, 2024.
If you do test positive, chlamydia is curable; antibiotics clear it reliably. The current first-line regimen and how the options compare are covered in chlamydia treatment.
If you think you've been exposed
Don't test the same day you had sex. The bacterium needs time to reach detectable levels, so testing too early can miss a real infection. Check the right timing in when to test after exposure, then get tested once you're inside the window.
When to see a clinician
See a clinician if a partner tells you they tested positive, if you've had a new partner and are due for screening, or if you notice abnormal discharge, burning with urination, pelvic pain, rectal pain or discharge, or bleeding between periods. For women, the warning signs of complications are worth knowing: untreated chlamydia can climb into the upper reproductive tract and cause pelvic inflammatory disease (infection and inflammation of the uterus and tubes), which can scar the fallopian tubes and lead to ectopic pregnancy (a pregnancy implanted outside the uterus, which is a medical emergency), infertility, and chronic pelvic pain. In men it can cause epididymitis (painful inflammation of the tube behind the testicle, occasionally affecting fertility). Either sex can develop reactive arthritis — joint inflammation that follows the infection. Chlamydia can also raise the risk of acquiring or transmitting HIV, so CDC recommends HIV testing for everyone diagnosed with it.