Chlamydia spreads primarily through vaginal, anal, or oral sex without a condom with someone who has the infection. What makes it so effective at spreading 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. With over 1.5 million cases reported in the United States in 2024 alone, it remains one of the most common sexually transmitted infections in the country.
Sexual Transmission Is the Primary Route
Chlamydia passes from person to person through contact with infected mucous membranes and the fluids they produce. During vaginal sex, the bacteria transfer between the lining of the cervix and the urethra. During anal sex, the rectal lining can become infected. Oral sex carries a lower but real risk, particularly for throat infections.
You don’t need to have penetrative sex to transmit it. Any contact between mucous membranes and infected genital fluids can be enough. The bacteria can also spread from one body site to another in the same person. Someone with a vaginal infection, for example, can develop a rectal infection without having had anal sex, simply through the natural spread of secretions.
Why Asymptomatic Carriers Drive the Spread
The single biggest reason chlamydia is so widespread is that most infections are silent. Three out of four women and half of men with chlamydia experience no noticeable symptoms. They feel fine, so they don’t get tested, don’t get treated, and continue having sex while infectious. The majority of cases go undiagnosed and unreported entirely.
Once exposed, the incubation period is roughly 7 to 21 days, though it can stretch to several months in some cases. During this entire window, and for as long as the infection goes untreated afterward, a person can transmit chlamydia to sexual partners. There’s no point at which an untreated infection “burns out” on its own and stops being contagious.
How the Bacteria Infects Your Cells
Chlamydia targets the thin, delicate cells lining your genital tract, rectum, throat, and eyes. The bacteria exist in two forms: a small, tough outer form designed for survival between hosts, and a larger, active form that reproduces inside your cells. When the tough outer form lands on a mucosal surface, it latches onto the cell in a two-step process, first loosely attaching, then locking on tightly to specific receptors.
Once attached, the bacteria inject proteins into the cell that rearrange its internal scaffolding, essentially forcing the cell to pull the bacteria inside. The bacteria then create a protective bubble within the cell, shielding themselves from the immune system. Inside this bubble, they convert to their active form, multiply, and eventually burst out to infect neighboring cells. This cycle of invasion, replication, and cell destruction is what causes tissue damage over time, even when you can’t feel it happening.
Transmission From Mother to Baby
A pregnant woman with chlamydia can pass the infection to her baby during vaginal delivery. As the baby moves through the birth canal, exposure to infected tissue and fluids can lead to eye infections (conjunctivitis) or pneumonia in the newborn.
Chlamydia during pregnancy also raises the risk of complications for the mother. A large population study in Washington State found that infected women were about 46% more likely to deliver preterm and 50% more likely to experience premature rupture of membranes compared to uninfected women. Babies born to mothers with chlamydia were also admitted to neonatal intensive care at a higher rate: 7.6% compared to 4.7% for babies of uninfected mothers. This is why routine chlamydia screening during pregnancy is standard practice.
Can It Spread Through Objects or Surfaces?
Chlamydia is a fragile organism outside the body. Lab studies have tested how long the bacteria survive on hard plastic surfaces, and under normal room conditions, half the samples were dead within 5 minutes. Complete desiccation, the point at which no viable bacteria remained, occurred at 45 minutes. In humid conditions the bacteria lasted longer, with some samples still viable at 3 hours, which is why researchers have noted a theoretical risk of eye infection from contaminated surfaces in warm, moist environments.
In practical terms, catching chlamydia from a toilet seat, shared towel, or doorknob is extremely unlikely. The bacteria need warm, moist mucosal tissue to survive and infect. Casual contact like hugging, shaking hands, sharing food, or sitting next to someone poses no risk.
Eye Infections Through Hand Contact
One non-sexual route worth knowing about is autoinoculation, which simply means transferring the bacteria from one part of your own body to another. If you touch infected genital secretions and then touch your eyes, chlamydia can infect the conjunctiva and cause a form of pink eye. This type of chlamydial eye infection is associated either with direct sexual contact or with touching the eyes after handling genital secretions. It’s uncommon but not rare, and it’s another reason handwashing matters during and after sexual activity.
How Condoms Reduce the Risk
Consistent condom use dramatically lowers the chance of transmission. In a study of people with known exposure to chlamydia, only 13.3% of consistent condom users became infected compared to 34.4% of those who used condoms inconsistently. That translates to roughly a 90% reduction in odds when condoms are used correctly every time.
The key word is “consistent.” Occasional condom use offers far less protection because chlamydia can transmit during any single unprotected encounter. Dental dams during oral sex and condoms during anal sex provide the same barrier benefit for those types of contact. For people who are sexually active with new or multiple partners, annual screening is one of the most effective tools for catching the silent infections that drive most transmission.