Chlamydia spreads primarily through sexual contact, including vaginal, anal, and oral sex. What makes it especially easy to transmit is that roughly 75% of infected women and 50% of infected men have no symptoms at all, meaning most people who pass it on don’t know they’re carrying it.
Vaginal and Anal Sex
The most common route of transmission is vaginal or anal intercourse with an infected partner. The bacteria, Chlamydia trachomatis, lives in mucous membranes and is carried in sexual fluids. Any unprotected penetrative contact creates direct mucosal-to-mucosal contact, which is exactly what the bacterium needs to transfer from one person to another. You don’t need to have an orgasm or exchange a large amount of fluid for transmission to occur.
Consistent condom use significantly lowers the risk. In one study of people with known chlamydia exposure, 13.3% of consistent condom users were diagnosed with the infection compared to 34.4% of those who used condoms inconsistently. That’s roughly a 90% reduction in odds when condoms are used every time.
Oral Sex
Chlamydia can infect the throat when a person performs oral sex on someone with a genital or rectal infection. Throat infections often cause no symptoms at all, which means people can unknowingly spread the bacteria back to a partner’s genitals during future oral contact. The CDC notes that having chlamydia in the throat may actually make it easier to pass the infection to others through oral sex. You can also be infected in more than one site at once, carrying the bacteria in both your throat and genitals simultaneously.
Throat chlamydia infections are generally considered less harmful than genital or rectal ones, but they still serve as a reservoir that keeps the bacteria circulating between partners.
Fingers, Hands, and Shared Sex Toys
Manual stimulation is an underappreciated route. Research has shown that fingers contaminated with chlamydia can transfer detectable amounts of the bacteria through touch. In lab conditions, about 20% of positive clinical specimens contained enough bacteria on a contaminated finger to potentially cause a new infection. This means touching an infected partner’s genitals and then touching your own eyes, genitals, or a partner’s body could move the bacteria to a new site.
The good news: hand washing reduced the amount of transferable bacteria by roughly 1,000-fold, making simple hygiene a surprisingly effective barrier. Shared sex toys follow the same logic. If a toy contacts infected fluids and is then used by another person without cleaning or a fresh barrier, it can carry the bacteria between partners.
Eye Infections From Genital Contact
Chlamydia can infect the eyes, causing a condition called chlamydial conjunctivitis. Most cases result from autoinoculation, meaning a person touches infected genital secretions (their own or a partner’s) and then touches their eyes. Some documented cases have also occurred from direct ejaculation into the eye during sex. In a few of those cases, the person had no genital chlamydia themselves, confirming that the bacteria came entirely from a partner’s fluids reaching the eye.
This mode of transmission is likely underreported because clinicians don’t routinely ask about it. If you develop a persistent, unexplained eye infection, especially one that doesn’t respond to standard treatments, chlamydia is worth considering.
During Childbirth
A pregnant person with untreated chlamydia can pass the infection to their baby during vaginal delivery. In one large study, the vertical transmission rate was about 30% when the mother’s infection was not identified and treated. Among mothers who tested positive and received treatment before delivery, that rate dropped to 1.5%.
For newborns who do contract the infection, the nasopharynx (the upper part of the airway behind the nose) is the most commonly affected site. Around 20% of infected newborns develop early-onset pneumonia or eye infections. Babies born to chlamydia-positive mothers had 3.6 times the risk of perinatal pneumonia compared to those born to uninfected mothers. This is why routine prenatal screening for chlamydia is standard practice.
How It Does Not Spread
Chlamydia is fragile outside the body. In lab testing on plastic surfaces, the bacteria lost viability within 45 minutes under normal room conditions, with half the samples already dead by the 5-minute mark. Humid environments extended survival somewhat (up to about 3 hours), but real-world scenarios like toilet seats, doorknobs, or swimming pools don’t create the conditions needed for transmission. You won’t catch chlamydia from casual contact, hugging, sharing food, or sitting on a surface an infected person used.
Why Asymptomatic Spread Matters
The central challenge with chlamydia is that the majority of infections are silent. When three out of four women and half of men feel perfectly fine while carrying the bacteria, the infection circulates through sexual networks without anyone realizing it. Symptoms, when they do appear, typically show up between one week and three months after exposure, but can take even longer. During that entire window, and potentially for months or years if the infection goes undetected, a person can transmit chlamydia to every new sexual partner.
This is why regular screening matters far more for chlamydia than for infections that reliably produce symptoms. If you’re sexually active with new or multiple partners, testing is the only reliable way to know your status. Chlamydia is easily curable with antibiotics, but it can only be treated if it’s found.