Chlamydia is a bacterial infection caused by Chlamydia trachomatis, and it is the most commonly reported sexually transmitted infection worldwide. An estimated 128.5 million new cases occur each year among adults aged 15 to 49. What makes chlamydia particularly tricky is that most people who have it don’t know it: 70 to 80% of infected women and up to 50% of infected men never develop noticeable symptoms.
How Chlamydia Spreads
Chlamydia passes from one person to another through vaginal, anal, or oral sex without a condom. The bacteria infect the cells lining the genitals, rectum, or throat, depending on the type of sexual contact involved. You don’t need to have many partners to contract it. A single unprotected encounter with someone carrying the infection is enough.
A pregnant person with chlamydia can also pass the infection to their baby during vaginal delivery. This is one reason screening during pregnancy matters. In newborns, chlamydia most often causes eye infections that appear 5 to 14 days after birth, starting as watery discharge that turns thick and sometimes bloody. Left untreated, it can damage the cornea or spread to the lungs and cause pneumonia.
What Happens Inside Your Body
Chlamydia is unusual among bacteria because it can only survive and reproduce inside your cells. The bacterium exists in two forms. The first is a small, tough particle called an elementary body that travels between people and latches onto cells in your genital tract, rectum, or throat. Once it enters a cell, it transforms into a larger form that hijacks the cell’s machinery to copy itself. Over roughly 36 to 50 hours, the cell fills with hundreds of new infectious particles. Eventually the cell ruptures, releasing those particles to infect neighboring cells and continue the cycle.
This intracellular lifestyle is part of why chlamydia can persist silently for months. Your immune system has a harder time detecting and clearing bacteria hiding inside your own cells.
Symptoms in Women
When symptoms do appear in women, they typically include abnormal vaginal discharge, a burning sensation during urination, and bleeding between periods or after sex. Some women experience lower abdominal pain or pain during intercourse. These symptoms can be mild enough to dismiss as a minor irritation or urinary issue, which is one reason infections often go undiagnosed.
Because 70 to 80% of women with chlamydia have no symptoms at all, routine screening is the primary way infections get caught. The U.S. Preventive Services Task Force recommends annual screening for all sexually active women aged 24 and younger, and for older women with risk factors like a new partner, multiple partners, or inconsistent condom use.
Symptoms in Men
Men are somewhat more likely to notice symptoms than women, though up to half still remain asymptomatic. When symptoms develop, they usually involve a clear or cloudy discharge from the penis and a burning feeling when urinating. Some men experience pain or swelling in one or both testicles, which signals that the infection has spread to the epididymis, the tube that carries sperm.
Rectal chlamydia, which can occur in anyone who has receptive anal sex, may cause rectal pain, discharge, or bleeding, though it’s also frequently silent.
Complications of Untreated Chlamydia
The real danger of chlamydia lies in what happens when it goes undetected and untreated for weeks or months. In women, the infection can travel upward from the cervix into the uterus and fallopian tubes, causing pelvic inflammatory disease (PID). Mathematical modeling estimates that about 22% of women with untreated chlamydia will develop PID. This inflammation can scar the fallopian tubes, leading to chronic pelvic pain, ectopic pregnancy (where a fertilized egg implants outside the uterus, a potentially life-threatening situation), and infertility.
In men, untreated infection can cause epididymitis, which, if severe or recurring, may also affect fertility. Chlamydia can additionally cause reactive arthritis in both sexes, a condition where the immune response to the infection triggers joint pain and swelling.
Testing and the Detection Window
Chlamydia is diagnosed with a simple urine sample or a swab of the vagina, rectum, or throat. The test detects the bacterium’s genetic material directly, making it highly accurate once enough bacteria are present.
Timing matters. If you were recently exposed, testing too early can produce a false negative. Most infections become detectable about one week after exposure. Waiting two weeks catches nearly all cases. If you test negative within a few days of a possible exposure, consider retesting after the two-week mark to be sure.
Treatment
Chlamydia is fully curable with antibiotics. The standard treatment is a seven-day course of oral antibiotics taken twice daily. Most people clear the infection completely with this single course. You should avoid sexual contact during treatment and for seven days after finishing the medication to prevent passing the infection to a partner.
All recent sexual partners need to be tested and treated as well, even if they have no symptoms. Otherwise, you can easily get reinfected. Reinfection is common and carries the same risks of complications as the original infection, so retesting about three months after treatment is a good idea.
Who Should Get Screened
Because chlamydia so often produces no symptoms, proactive screening is the most effective way to prevent complications. Current guidelines recommend annual screening for all sexually active women aged 24 and younger. Women 25 and older should be screened if they have risk factors: a new sexual partner, more than one partner, a partner who has other partners, inconsistent condom use outside a monogamous relationship, a previous STI, or a history of exchanging sex for drugs or money.
For men, there is no universal screening recommendation, though men who have sex with men are generally advised to screen annually or more frequently based on their number of partners. Anyone who learns a sexual partner has tested positive should get tested regardless of symptoms or gender.