Chlamydia spreads primarily through sexual contact, when the bacteria pass from one person’s infected mucous membranes to another’s. This includes vaginal, anal, and oral sex. It can also pass from mother to baby during childbirth. You cannot get chlamydia from toilet seats, towels, or casual contact.
Sexual Contact Is the Primary Route
The bacterium that causes chlamydia infects the thin, moist tissue (epithelium) lining the genitals, rectum, and throat. During vaginal or anal sex, direct contact between these tissues allows the bacteria to transfer from one partner to another. Vaginal sex is the most common route, but anal sex carries a similar level of risk, and a person can develop a rectal chlamydia infection even without genital symptoms.
Oral sex is a less efficient route, but it’s still possible. Giving oral sex to an infected partner can lead to a chlamydia infection in the throat, and having a throat infection may make it easier to pass chlamydia to others during oral sex. The CDC notes that it’s difficult to isolate the exact risk from oral sex alone, because most people who have oral sex also have vaginal or anal sex with the same partners.
The per-partnership transmission probability gives a sense of how contagious the infection is. A large evidence synthesis published in the International Journal of Epidemiology estimated that male-to-female transmission runs around 32 to 35% per sexual partnership, while female-to-male transmission is lower, estimated between roughly 5 and 21% depending on the population studied. These are per-partnership figures, not per-act, meaning the cumulative risk rises with repeated unprotected encounters.
Most People Have No Symptoms
One of the reasons chlamydia spreads so effectively is that most carriers don’t know they have it. About 75% of women and 50% of men with chlamydia experience no symptoms at all. That means a person can transmit the infection across multiple sexual encounters, sometimes over weeks or months, without any visible signs prompting them to get tested or avoid sex. This is why routine screening matters, particularly for sexually active people under 25 and anyone with new or multiple partners.
Mother-to-Child Transmission During Birth
A pregnant person with untreated chlamydia can pass the bacteria to their baby during vaginal delivery. The infant picks up the infection while passing through the birth canal. In one cohort study, about 21% of infants born to mothers with chlamydia became infected. The most common complications in newborns are conjunctivitis (an eye infection) and pneumonia. Infected babies had 3.6 times the risk of developing perinatal pneumonia compared to uninfected infants, and roughly two-thirds of infected newborns in the study showed respiratory distress shortly after birth.
Treatment makes a dramatic difference. When mothers who tested positive received antibiotics before delivery, the vertical transmission rate dropped to just 1.5%. Preterm infants are especially vulnerable, with infection rates nearly double those of full-term babies in the same study.
How Chlamydia Does Not Spread
Chlamydia bacteria need the warmth and moisture of human mucous membranes to survive. They die quickly on dry surfaces like toilet seats, doorknobs, and shared towels. By the time another person touches that surface, the bacteria are no longer viable. You also cannot catch chlamydia from hugging, sharing food or drinks, swimming pools, or hot tubs. The infection requires direct mucous membrane contact, which in practical terms means sexual activity or passage through the birth canal.
How Condoms Reduce the Risk
Consistent, correct condom use reduces the risk of chlamydia transmission by about 60%, based on a study of urban adolescents published in JAMA Pediatrics. That’s meaningful protection, but it’s not as high as the reduction condoms provide for HIV. The gap exists because chlamydia can infect areas that a condom doesn’t fully cover, and because real-world condom use is rarely perfect for every single encounter. Still, condoms remain one of the most practical tools for lowering your risk, especially when combined with regular testing.
How Long You’re Contagious After Treatment
Once you start antibiotics, you should avoid sex for at least seven days. If you’re on a single-dose treatment, the seven-day wait starts from the day you take it. If you’re on a week-long course, wait until you’ve finished all the medication and any symptoms have cleared. Having sex before that window closes risks passing the infection to a partner even though you’re already being treated.
Partners need treatment too. If only one person in a sexual partnership gets treated, the untreated partner can simply re-infect them, a pattern sometimes called “ping-pong” transmission.
When a Test Can Detect It
If you think you’ve been exposed, testing too early can produce a false negative. Chlamydia testing, typically a urine sample or a swab of the vagina, rectum, or throat, is reliable about one week after exposure in most cases. Waiting two weeks catches nearly all infections. If you test negative at one week but still have concerns, a follow-up test at the two-week mark provides stronger reassurance.