Chlamydia spreads through vaginal, anal, or oral sex without a condom with someone who has the infection. It can also pass from a pregnant person to their baby during childbirth. What makes chlamydia especially easy to spread is that most people who have it don’t know it: at least 70% of women and 50% of men with genital chlamydia have no symptoms at the time of diagnosis.
How the Bacteria Enter Your Body
Chlamydia is caused by the bacterium Chlamydia trachomatis, which targets the moist, delicate tissue lining your genitals, rectum, throat, and eyes. When this tissue comes into contact with infected bodily fluids during sex, the bacteria latch onto cells in a two-step process: first loosely attaching to the cell surface, then binding tightly to specific receptors. Once attached, the bacteria trigger your cells to pull them inside, where they set up shop in a protected compartment and begin multiplying. This whole process happens without causing obvious damage at first, which is one reason so many infections go unnoticed.
Sexual Transmission Routes
Any unprotected sexual contact that involves mucous membranes can transmit chlamydia. Vaginal sex is the most common route, but anal and oral sex carry real risk too. You can get a rectal infection either through receptive anal sex or when the bacteria spread from a vaginal infection to the rectum on their own. Throat infections from oral sex are less common but do occur.
The risk isn’t equal in both directions. Modeling studies using population survey data estimate that the probability of transmission across an entire sexual partnership (not just one encounter) is roughly 32 to 35% from male to female, and somewhere between 5 and 21% from female to male. Per individual sex act, the risk is lower, with estimates ranging from about 4% to 15%. The higher male-to-female rate likely reflects the anatomy involved: the cervix has a larger surface area of vulnerable tissue than the urethra.
Ejaculation isn’t required for transmission. The bacteria live in infected tissue and secretions, so any genital contact that exchanges fluids or involves direct mucous membrane contact can be enough.
Why Asymptomatic Spread Is So Common
Chlamydia’s ability to spread silently is its defining feature from a public health standpoint. Seven out of ten women with cervical chlamydia have no symptoms or only mild ones that wouldn’t prompt a doctor visit. About half of men with the infection are similarly symptomless. Because people don’t know they’re infected, they continue having sex and passing the bacteria along without realizing it.
A person becomes contagious as soon as the infection takes hold and stays contagious until they complete treatment. If symptoms do appear, they typically show up within 7 to 21 days after exposure, with 7 to 14 days being most common. But since the majority of cases never produce noticeable symptoms, waiting for signs of infection is not a reliable strategy. Regular screening is the only way to catch it early.
Transmission During Pregnancy and Childbirth
A pregnant person with untreated chlamydia can pass the infection to their baby during vaginal delivery. As the baby moves through the birth canal, bacteria can infect the baby’s eyes, potentially causing conjunctivitis (an eye infection that appears in the first weeks of life), or enter the lungs, leading to pneumonia in the first few months.
Chlamydia during pregnancy also raises risks for the pregnancy itself. Research from a large population-based study found that infected women had a 46% higher risk of preterm delivery and a 50% higher risk of their water breaking early compared to uninfected women. These risks are why prenatal screening for chlamydia is standard practice.
Eye Infections From Hand Contact
Chlamydia can infect the eyes when contaminated fluids are transferred by hands, shared towels, or other items that touch the face. This means you can give yourself an eye infection by touching your genitals and then your eyes without washing your hands. Chlamydial eye infections cause redness, discharge, and swelling, and in parts of the world where a specific strain circulates widely (causing the disease called trachoma), repeated eye infections are a leading cause of preventable blindness.
Shared Sex Toys and Surfaces
Chlamydia trachomatis cannot survive long outside the human body, which limits indirect transmission significantly. The chance of catching chlamydia from a toilet seat, swimming pool, shared utensils, or casual physical contact like hugging or handshaking is essentially zero. The bacteria need warm, moist mucous membranes to survive, and hard, dry surfaces kill them quickly.
Sex toys are a narrow exception. If a toy is used by an infected person and then immediately shared without cleaning, transmission is possible, particularly with insertable toys made from porous materials like neoprene or vinyl, which can trap bodily fluids in tiny surface holes. Cleaning toys thoroughly before and after each use, or using a fresh condom over them, eliminates this risk. Letting a toy sit unused for around 24 hours also gives the bacteria enough time to die off.
What Doesn’t Spread Chlamydia
You cannot get chlamydia from kissing, hugging, sharing food or drinks, coughing, sneezing, or sitting on a toilet seat. The bacteria are fragile outside the body and need direct contact with mucous membranes to establish an infection. Living in the same household as someone with chlamydia poses no risk through normal daily activities. The infection requires the kind of intimate contact that brings mucous membranes or infected secretions together.
Reducing Your Risk
Condoms, when used consistently and correctly, are the most effective way to reduce chlamydia transmission during sex. They create a barrier that prevents the bacteria from reaching vulnerable tissue. For oral sex, dental dams serve the same purpose. Neither method is 100% effective, since skin-to-skin contact around areas the condom doesn’t cover can still theoretically transfer bacteria, but condoms dramatically lower the odds.
Because chlamydia so often has no symptoms, screening is critical. Sexually active women under 25 and anyone with new or multiple partners benefit from annual testing. If you test positive, all recent sexual partners need to be notified and treated to prevent reinfection. Chlamydia is curable with antibiotics, but a past infection doesn’t protect you from getting it again, so repeat exposure means repeat risk.