Chlamydia spreads through vaginal, anal, or oral sex with someone who has the infection. It passes through direct contact with infected genital fluids, and ejaculation does not need to occur for transmission. This makes it one of the most easily spread sexually transmitted infections, with over 1.5 million cases reported in the United States each year.
Sexual Contact Is the Primary Route
The bacterium that causes chlamydia lives in genital fluids, including vaginal secretions, pre-ejaculate, and semen. Any unprotected sexual contact that involves these fluids can transmit the infection. Vaginal sex is the most common route, but anal sex carries the same risk. Receptive anal sex can lead to a rectal infection directly, and chlamydia can also spread from the vagina to the rectum without anal sex occurring at all.
Oral sex is a less efficient but confirmed route. Performing oral sex on an infected partner can lead to a throat infection. While pharyngeal chlamydia may cause fewer problems than genital or rectal infections, it can still be passed to a partner’s genitals through oral contact, creating a cycle of transmission that many people don’t consider.
Skin-to-skin genital contact, even without penetration, can also transmit the infection. The bacteria can be present on mucosal surfaces, so any direct genital-to-genital touching poses some risk.
Most People With Chlamydia Have No Symptoms
This is the single most important fact about chlamydia transmission: 75% of infected women and 50% of infected men never develop noticeable symptoms. That means the majority of people spreading chlamydia have no idea they carry it. They feel fine, see nothing unusual, and unknowingly pass the infection to sexual partners.
When symptoms do appear, they typically show up one to three weeks after exposure, though some people don’t notice anything for months. For women, symptoms can include unusual vaginal discharge, bleeding between periods, or pain during urination. Men may notice discharge from the penis, burning while urinating, or testicular discomfort. Rectal infections can cause pain, discharge, or bleeding regardless of sex. But again, the absence of symptoms is far more common than their presence, which is why routine screening matters so much for sexually active people.
Transmission During Childbirth
Chlamydia can pass from mother to baby during vaginal delivery. As the infant moves through the birth canal, exposure to the bacteria can cause neonatal conjunctivitis, an eye infection that develops in the first few weeks of life. It can also lead to a respiratory infection in the newborn’s lungs. Untreated chlamydia during pregnancy has additionally been linked to premature rupture of membranes, preterm delivery, and low birth weight. This is why prenatal screening for chlamydia is standard practice.
How It Does Not Spread
Chlamydia cannot spread through casual contact. The bacteria require the specific warmth and moisture of the human body to survive and die quickly on external surfaces. You cannot get chlamydia from toilet seats, shared towels, swimming pools, hot tubs, or doorknobs. Hugging, handshaking, sharing food or drinks, and sitting next to someone are all completely safe. The infection requires direct sexual or mucosal contact to transmit.
Condoms Significantly Reduce Risk
Consistent condom use provides strong protection. In a study of people with known chlamydia exposure, only 13.3% of those who used condoms consistently became infected, compared to 34.4% of those who used them inconsistently. That’s roughly a 90% reduction in odds when condoms are used correctly every time. Dental dams during oral sex also reduce risk, though they’re used far less often in practice.
Condoms aren’t perfect because chlamydia can infect areas not covered by the barrier, but they remain the most effective tool for reducing transmission during sex.
Reinfection After Treatment
Getting treated for chlamydia does not protect you from getting it again. Reinfection is common, particularly when a sexual partner wasn’t treated at the same time. If you complete your antibiotics but your partner still carries the bacteria, the infection will pass right back the next time you have unprotected sex.
The CDC recommends retesting three months after treatment to catch repeat infections. Both partners need to be treated, and you should avoid sexual contact until treatment is complete for both of you. Repeated chlamydia infections increase the risk of complications like pelvic inflammatory disease in women, which can affect fertility over time.
Who Faces the Highest Risk
Anyone who is sexually active can get chlamydia, but certain factors increase the likelihood. Having multiple sexual partners, not using condoms, and having a partner who has other sexual partners all raise the risk. Young adults under 25 account for a disproportionate share of cases, which is why annual screening is recommended for all sexually active women in that age group and for anyone with new or multiple partners.
In the U.S., chlamydia remains the most commonly reported bacterial STI, though cases have declined 8% since 2023. That drop likely reflects both increased screening and treatment rather than a true decrease in sexual transmission, since the high rate of asymptomatic cases means many infections still go undetected.