Chlamydia spreads through vaginal, anal, or oral sex with someone who has the infection. Ejaculation doesn’t need to happen for transmission to occur. Between 50% and 70% of people with chlamydia never notice symptoms, which means many infections are passed along by people who have no idea they’re carrying the bacteria.
Sexual Contact Is the Primary Route
The bacterium that causes chlamydia lives in infected genital, rectal, and throat secretions. Any unprotected sexual contact that involves these areas can transmit it. Vaginal and anal sex carry the highest risk, but oral sex can also spread the infection, particularly to the throat.
Chlamydia can also infect the rectum in two ways: through receptive anal sex, or by spreading from an existing vaginal infection to the rectal area without any anal contact at all. This means someone can have a rectal chlamydia infection without ever having had anal sex.
How Likely Is Transmission From a Single Partner
The odds of catching chlamydia aren’t a coin flip, but they’re not negligible either. A large evidence synthesis published in the International Journal of Epidemiology estimated that the male-to-female transmission probability over the course of a sexual partnership is roughly 32% to 35%. Female-to-male transmission is lower, though estimates vary widely, ranging from about 5% to 21% depending on the population studied. These figures represent entire partnerships rather than single encounters, so the per-act risk is lower, but the cumulative risk builds quickly with repeated unprotected contact.
Touching Can Spread It to the Eyes
Chlamydia doesn’t only infect the genitals. One underappreciated route is hand-to-eye transfer. If you touch infected genital secretions, whether your own or a partner’s, and then touch your eyes, the bacteria can cause chlamydial conjunctivitis. Most cases of chlamydia-related eye infections are thought to result from this kind of self-inoculation rather than from direct contact with ejaculate. The resulting eye infection causes redness, discharge, and swelling that can persist for weeks if untreated.
Transmission During Childbirth
A pregnant person with an untreated chlamydia infection can pass the bacteria to their baby during delivery. In one study of infants born vaginally to infected mothers, nearly half tested positive for the bacterium afterward. Cesarean delivery reduces the risk but doesn’t eliminate it entirely: the bacteria were still isolated from some infants born by C-section, including cases where the membranes hadn’t ruptured beforehand. In newborns, chlamydia can cause eye infections and pneumonia, which is why prenatal screening is standard practice.
You Can’t Catch It From a Toilet Seat
The chlamydia bacterium cannot survive outside the human body for more than a very brief period. It needs the warm, moist environment of mucous membranes to stay viable. There is virtually zero chance of contracting chlamydia from a toilet seat, swimming pool, shared towel, or casual skin-to-skin contact like hugging or handshaking. The infection requires direct contact with infected secretions during sexual activity or, in rare cases, transfer by hand to a mucous membrane like the eye.
Why So Many Cases Go Unnoticed
The reason chlamydia spreads so effectively is that most people who carry it feel completely fine. Between 50% and 70% of infected individuals never develop noticeable symptoms regardless of sex. When symptoms do appear, they typically show up one to three weeks after exposure and can include unusual genital discharge, burning during urination, or rectal pain. But the absence of symptoms doesn’t mean the infection is harmless. Left untreated, chlamydia can cause pelvic inflammatory disease, chronic pain, and fertility problems.
This silent nature also creates a reinfection cycle. If your sexual partner isn’t tested and treated at the same time you are, you can catch the same infection again almost immediately.
Who Is Most at Risk
Chlamydia is most common among younger people, with rates peaking in the late teens and twenties. Beyond age, the biggest risk factors are straightforward: having unprotected sex and having multiple sexual partners. Certain strains that cause a more aggressive form of infection called lymphogranuloma venereum have seen a resurgence specifically among men who have sex with men. Because most infections produce no symptoms, routine screening is recommended for sexually active women under 25 and for anyone with new or multiple partners.
How Condoms and Testing Reduce Risk
Consistent condom use dramatically cuts the risk of transmission. In one study of people with known chlamydia exposure, 13.3% of those who used condoms consistently became infected compared to 34.4% of inconsistent users. That translates to roughly a 90% reduction in odds. The key word is “consistent,” meaning every time, from start to finish. Occasional use offers far less protection.
If you’ve had unprotected sex and want to get tested, timing matters. A chlamydia test can detect the infection as early as one week after exposure in most cases, and waiting two weeks catches nearly all infections. Testing is done with a simple urine sample or a swab of the vagina, rectum, or throat depending on the type of contact. The infection is curable with a short course of antibiotics, making early detection one of the most effective tools for stopping its spread.