Chlamydia is a bacterial infection that invades your cells, quietly replicates, and can damage your reproductive system if left untreated. Over 1.5 million cases were reported in the U.S. in 2024 alone, making it the most commonly reported bacterial sexually transmitted infection in the country. What makes chlamydia particularly harmful is that most people who have it don’t know it, because the infection often causes no symptoms at all while it silently spreads and causes internal damage.
How Chlamydia Works Inside Your Body
Chlamydia isn’t like most bacteria. It can’t survive or reproduce on its own. Instead, it has a two-phase life cycle designed to get inside your cells and hijack them. The infectious form, called an elementary body, is small and tough enough to survive outside a cell, but it’s metabolically inactive. Think of it as a seed. Once it attaches to a cell lining your genitals, throat, or rectum, it tricks the cell into absorbing it.
Once inside, the bacteria transforms into its active, replicating form. Within about 16 hours of infecting a cell, it begins multiplying inside a protective bubble called an inclusion. The cell essentially becomes a factory. Eventually the cell bursts, releasing hundreds of new infectious particles that spread to neighboring cells. This cycle of invasion, replication, and cell destruction is what drives the inflammation and tissue damage chlamydia causes over time.
Symptoms in Women
Most women with chlamydia have no symptoms, which is why routine screening matters. When symptoms do appear, they typically show up 1 to 3 weeks after exposure, though the incubation period can stretch to several months in some cases.
The most common signs include abnormal vaginal discharge (sometimes with a strong smell), a burning sensation when urinating, and bleeding between periods. If the infection has already started spreading beyond the cervix, you may notice lower abdominal pain, pain during sex, nausea, or fever. These are warning signs that the infection is moving deeper into the reproductive tract.
Symptoms in Men
Men are somewhat more likely to notice something is wrong, but many still have no symptoms. When signs do appear, they typically include painful urination and a discharge from the penis, which can range from clear and watery to thicker and cloudy. Some men also experience testicular pain or swelling if the infection spreads to the epididymis, the tube that stores and carries sperm.
Infections Beyond the Genitals
Chlamydia doesn’t only infect the genitals. It can also take hold in the rectum and throat, depending on how it was transmitted. Rectal chlamydia may cause pain, discharge, or bleeding, but it frequently causes no symptoms at all. Throat infections are also typically silent. These extragenital infections are easily missed because standard screening often focuses on genital sites only, and many people don’t think to mention oral or anal exposure.
What Happens If It Goes Untreated
This is where chlamydia does its most serious damage. In women, untreated chlamydia can climb from the cervix into the uterus and fallopian tubes, triggering pelvic inflammatory disease (PID). About 10 to 15 percent of women with untreated chlamydia will develop PID. The infection causes inflammation and scarring in the fallopian tubes, and that scarring can be permanent. The consequences include chronic pelvic pain, infertility, and a higher risk of ectopic pregnancy, where a fertilized egg implants outside the uterus. Ectopic pregnancies are medical emergencies.
Even without obvious PID symptoms, chlamydia can cause what’s known as “silent” infection in the upper reproductive tract. The damage accumulates without pain or fever, which means some women don’t discover the harm until they have difficulty getting pregnant.
In men, untreated chlamydia can lead to epididymitis and, less commonly, to reduced fertility. It can also cause reactive arthritis in both sexes, a condition where the immune response to the infection triggers joint pain, eye inflammation, and urinary symptoms.
Risks During Pregnancy
Chlamydia poses real dangers during pregnancy and childbirth. Without treatment, the bacteria can pass from mother to baby during vaginal delivery at alarmingly high rates, with some estimates as high as 50 to 70 percent. Among infants born to mothers with active, untreated chlamydia, 30 to 50 percent develop eye infections (conjunctivitis), and 10 to 20 percent develop pneumonia. Both are treatable, but preventable screening and treatment during pregnancy eliminates the risk entirely.
How It’s Detected
The standard test for chlamydia is a nucleic acid amplification test, which detects the bacteria’s genetic material. It’s highly accurate, with sensitivity above 90 percent and specificity at 99 percent or higher. That means false negatives are uncommon and false positives are rare. For women, a vaginal swab performs just as well as a cervical swab, and self-collected swabs are an option at many clinics. For men, a urine sample works as well as, or better than, a urethral swab. Rectal and throat swabs can also be tested when needed, though these sites aren’t always included in routine screening unless specifically requested.
These tests detect 20 to 50 percent more infections than older methods like culture, which is why they’ve become the universal standard.
How It’s Treated
Chlamydia is curable with antibiotics. The current recommended treatment is a seven-day course of an oral antibiotic taken twice daily. For people who are unlikely to complete a full week of medication, a single-dose alternative exists, though it’s slightly less effective for rectal infections and may require a follow-up test to confirm the infection cleared.
You should avoid sexual contact until treatment is finished and any partners have been treated as well. Reinfection is common, not because the antibiotics failed, but because people are re-exposed by an untreated partner. Retesting about three months after treatment is recommended for this reason. Having chlamydia once doesn’t make you immune to getting it again.
Why Screening Matters
Because chlamydia so often produces no symptoms, the only reliable way to catch it early is through screening. The infection is most common in sexually active people under 25, and annual screening is recommended for women in that age group, as well as for anyone with new or multiple sexual partners. The gap between infection and noticeable harm can be months or years, which means the window for easy, consequence-free treatment is wide, but only if the infection is actually found.