Chlamydia spreads through sexual contact with an infected person, including vaginal, anal, and oral sex. It does not require ejaculation to pass from one person to another. What makes chlamydia particularly easy to catch 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, meaning they can pass the infection along without realizing anything is wrong.
Sexual Contact Is the Primary Route
The bacterium that causes chlamydia lives in the mucous membranes of the genitals, rectum, and throat. Any sexual activity that brings these tissues into contact can transmit the infection. Vaginal and anal sex carry the clearest risk, but oral sex can also spread chlamydia, particularly when performing oral sex on a partner who has a genital or rectal infection. Throat infections from chlamydia are possible, and a person with chlamydia in the throat may be able to pass it to a partner’s genitals through oral sex.
Chlamydia can infect multiple body sites at once. Someone might have a rectal infection from anal sex and a genital infection simultaneously, sometimes without symptoms at either site. This is one reason screening based only on symptoms misses so many cases.
You Cannot Get It From Casual Contact
The bacteria that cause chlamydia cannot survive outside the human body for long. You will not get chlamydia from toilet seats, swimming pools, shared towels, doorknobs, or hugging and kissing. It requires direct contact with infected mucous membranes or the fluids they produce. This is a common concern, but the biology simply doesn’t support environmental transmission.
Why So Many People Spread It Unknowingly
The reason chlamydia is the most commonly reported bacterial sexually transmitted infection comes down to its silent nature. Most infected people feel completely fine. When symptoms do appear, they typically show up one to three weeks after exposure, but many infections never produce noticeable signs at all. A person can carry and transmit chlamydia for months without any clue.
This creates a cycle: someone gets treated, returns to an untreated partner, and gets reinfected. A mathematical modeling study published in Sexually Transmitted Infections estimated that without partner treatment, the probability of reinfection from an untreated partner is about 19%. This is why both partners need treatment at the same time, even if one feels perfectly healthy.
Factors That Increase Your Risk
Age is the single strongest predictor. Chlamydia prevalence is highest among adolescents and young adults under 25. Part of this is biological: younger women are more likely to have a type of cervical tissue that is more susceptible to infection. Part of it is behavioral, including having new or multiple sexual partners and inconsistent condom use.
The numbers bear this out. Among sexually active people who reported one partner in the past year, prevalence was 1.4%. Among those with two or more partners, it rose to 3.2%. Other factors associated with higher rates include lower income, earlier age at first sexual experience, and not being in a long-term cohabiting relationship. None of these factors guarantee infection, but they shift the odds.
Inconsistent condom use is a major contributor. Some younger people may lack the power in a relationship to insist on condom use, which is a contextual risk factor that goes beyond individual decision-making.
Who Should Get Tested
Because chlamydia so often produces no symptoms, routine screening is the main way infections get caught. The CDC recommends annual chlamydia screening for all sexually active women under 25, and for women 25 and older who have risk factors like a new partner, multiple partners, or a partner with an STI. Anyone with a cervix falls under these guidelines regardless of gender identity.
For men who have sex with men, the recommendation is at least annual testing at all sites of sexual contact (urethra, rectum), with more frequent testing every three to six months for those at higher risk. For heterosexual men at low risk, routine screening isn’t strongly recommended, though it may be offered in high-prevalence settings like STI clinics or correctional facilities.
After treatment, retesting about three months later is recommended to catch reinfections, which are common. Pregnant women under 25 should be tested early in pregnancy and again in the third trimester, since untreated chlamydia can cause complications during delivery.
What Happens if It Goes Untreated
Chlamydia is easily cured with antibiotics, but untreated infections can cause serious problems. In women, the bacteria can travel from the cervix into the uterus and fallopian tubes, causing pelvic inflammatory disease. This can lead to chronic pelvic pain, scarring of the reproductive tract, and difficulty getting pregnant. In men, untreated chlamydia can cause painful inflammation in the tube that carries sperm, though this is less common.
Rectal chlamydia, if untreated, can cause pain, discharge, and inflammation. Throat infections tend to cause fewer problems and may clear on their own, though they can still be passed to partners during that time. The core issue with any untreated chlamydia infection is not just the potential damage to your own body but the ongoing, invisible chain of transmission to others.