Chlamydia is a bacterial sexually transmitted infection caused by the organism Chlamydia trachomatis. It is the most commonly reported bacterial STI in the United States, with over 1.5 million cases reported in 2024 alone. Most people who have it don’t know it: roughly 75% of infected women and 50% of infected men experience no symptoms at all, which is why routine screening matters so much.
How the Infection Works
Chlamydia has an unusual two-phase lifecycle. The bacterium exists in two forms: a tough, spore-like form that survives outside cells and initiates infection, and a fragile replicating form that lives inside your cells. Once the infectious form attaches to cells lining the genital tract, rectum, or throat, it gets pulled inside and transforms into its replicating version. It then hijacks the cell’s own nutrients to fuel its growth, copying itself multiple times before converting back into the infectious form. The new bacteria exit by either budding off from the cell or rupturing it entirely, then go on to infect neighboring cells or spread to a new person.
This intracellular lifestyle is part of what makes chlamydia so sneaky. Because the bacteria hide inside your own cells, the immune system often mounts a weak or delayed response, allowing the infection to persist for months without obvious signs.
Symptoms in Women and Men
When symptoms do appear, they typically show up several weeks after exposure. In women, the most common signs are abnormal vaginal discharge and a burning sensation when urinating. In men, symptoms can include discharge from the penis, burning during urination, and occasionally pain or swelling in one or both testicles.
Chlamydia can also infect the rectum, either through receptive anal sex or, in women, by spreading from the genital tract. Rectal infections often cause no symptoms but can produce rectal pain, discharge, or bleeding. Throat infections are possible through oral sex, though they tend to cause fewer problems than genital or rectal infections. However, having chlamydia in the throat can make it easier to pass the infection to partners during oral sex.
How It Spreads
Chlamydia spreads through vaginal, anal, and oral sex with an infected partner. It does not require ejaculation to transmit. Any contact between mucous membranes (the moist tissue lining the genitals, rectum, or throat) can pass the bacteria. The infection can also be transmitted from a pregnant person to their baby during vaginal delivery, potentially causing eye infections or pneumonia in the newborn.
The exact risk of transmission from any single sexual encounter is difficult to pin down, since it depends on the type of sex, the site of infection, and individual factors. What is clear is that because most carriers have no symptoms, people frequently pass chlamydia to partners without realizing they’re infected.
Complications of Untreated Infection
Left untreated, chlamydia can cause serious reproductive damage, particularly in women. The most significant risk is pelvic inflammatory disease (PID), an infection of the uterus, fallopian tubes, or ovaries. A long-term study following women from 2008 to 2022 found that those who tested positive for chlamydia developed PID at roughly twice the rate of women who tested negative. PID can cause chronic pelvic pain and scarring in the fallopian tubes, which blocks eggs from reaching the uterus.
That scarring can lead to tubal factor infertility. In the same study, women with a history of chlamydia were nearly three times more likely to be diagnosed with tubal factor infertility compared to those without. It can also increase the risk of ectopic pregnancy, where a fertilized egg implants in a fallopian tube instead of the uterus, a potentially life-threatening emergency.
In men, untreated chlamydia occasionally spreads to the tube that carries sperm from the testicle, causing pain and swelling. While this rarely leads to infertility in men, it can be quite painful.
Testing and Diagnosis
Chlamydia testing uses a method called nucleic acid amplification, which detects the bacterium’s genetic material. These tests are highly accurate, with sensitivity above 90% and specificity at 99% or higher. They catch 20% to 50% more infections than older testing methods.
For women, a vaginal swab is the preferred sample. Self-collected vaginal swabs are just as accurate as those collected by a clinician, which means many testing programs now allow you to collect your own sample in a clinic bathroom or even at home with a mail-in kit. For men, a urine sample works as well as or better than a urethral swab. Rectal and throat infections are tested with swabs at those sites.
Who Should Get Screened
Because chlamydia so often produces no symptoms, screening is the main way infections get caught. The CDC recommends annual screening for all sexually active women under 25. Women 25 and older should be screened if they have risk factors like a new partner, multiple partners, a partner who has other partners, inconsistent condom use outside a monogamous relationship, or a previous STI.
Men who have sex with men should be screened at least once a year at all sites of sexual contact, regardless of condom use. Those on PrEP, living with HIV, or with multiple partners should be tested every three to six months. 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. Pregnant women under 25 should be tested, as should pregnant women over 25 with risk factors, since treating the infection before delivery prevents transmission to the baby.
Treatment
Chlamydia is curable with antibiotics. The standard treatment is a seven-day course of doxycycline taken twice daily. For people who can’t take doxycycline, a single-dose alternative is available. Pregnant individuals receive a different antibiotic that’s safe during pregnancy.
You should avoid sex for seven days after completing treatment (or seven days after a single-dose regimen) to prevent passing the infection. Retesting is recommended about three months after treatment, since reinfection is common, often from an untreated partner. Having chlamydia once does not protect you from getting it again. If your partner isn’t treated at the same time, you can easily pass the infection back and forth.
Reducing Your Risk
Consistent condom use significantly reduces but does not eliminate the risk of chlamydia transmission, since the bacteria can infect areas not covered by a condom. Limiting the number of sexual partners lowers your overall exposure. The most effective prevention strategy is regular screening combined with prompt treatment, which breaks the chain of transmission even when symptoms are absent.