Chlamydia spreads through vaginal, anal, or oral sex without a condom with someone who has the infection. It is the most commonly reported bacterial sexually transmitted infection in the United States, with roughly 1.5 million cases reported in 2024 alone. What makes chlamydia especially easy to catch is that most people who have it don’t know they’re infected: about 75% of women and 50% of men with chlamydia never develop noticeable symptoms.
Sexual Contact Is the Primary Route
Chlamydia is caused by a bacterium that thrives in the warm, moist lining of the genitals, rectum, and throat. Any sexual contact that brings these tissues together can transmit the infection. Vaginal sex is the most common route, but anal and oral sex carry real risk as well. You don’t need to have many partners or engage in “risky” behavior in any dramatic sense. A single unprotected encounter with an infected person is enough.
Rectal infections deserve a specific mention. You can get chlamydia in the rectum through receptive anal sex, but the bacteria can also spread from an existing vaginal infection to the rectum without anal sex ever occurring. This means rectal chlamydia sometimes shows up in people who wouldn’t expect it.
Why So Many People Spread It Without Knowing
The reason chlamydia is so widespread comes down to its silent nature. Three out of four women and half of men who are infected feel perfectly fine. They have no discharge, no burning, no pain. Because they feel healthy, they don’t get tested, and the bacteria keeps circulating to new partners. This is why routine screening matters so much for sexually active people, particularly women under 25, who have the highest infection rates.
When symptoms do appear, they typically show up within one to three weeks after exposure. In women, that can mean unusual vaginal discharge, burning during urination, or bleeding between periods. In men, it usually presents as a discharge from the penis, burning with urination, or occasional pain and swelling in one testicle. But the absence of these signs tells you nothing about whether the infection is present.
How the Bacteria Takes Hold
Once the bacterium reaches the cells lining the genitals, throat, or rectum, it uses a surprisingly clever strategy. It enters a cell in a dormant, compact form designed purely for attachment and entry. Within hours, it hijacks the cell’s own machinery to avoid being destroyed by the immune system. The bacterium essentially hides inside a protective bubble within the cell, positioning itself near structures the cell uses for shipping nutrients, and taps into that supply line.
Over the next 24 to 36 hours, it shifts into an active form that divides rapidly. After about 40 to 60 hours total, the host cell bursts open, releasing a new wave of infectious particles that invade neighboring cells. This cycle repeats, allowing the infection to establish itself and persist for months or even years if untreated.
Transmission During Childbirth
Chlamydia can also pass from mother to baby during vaginal delivery. Infants born to mothers with an active genital infection are at risk for developing eye infections (conjunctivitis) and pneumonia in the weeks after birth. This is one reason pregnant women are routinely screened for chlamydia. Treating the infection before delivery prevents these complications.
What Doesn’t Give You Chlamydia
Chlamydia is not spread through casual contact. You won’t get it from a toilet seat, a swimming pool, sharing towels, or hugging. The bacterium can technically survive on surfaces for two to three hours under humid conditions, but this is not a meaningful transmission route. The bacterium needs direct contact with susceptible mucosal tissue to cause infection, and that essentially requires sexual contact or passage through the birth canal.
Kissing, sharing food, and sitting next to someone on a bus are all perfectly safe. Chlamydia is not airborne, and it does not live in saliva in any infectious quantity.
Factors That Increase Your Risk
Certain patterns raise the odds of contracting chlamydia. Having unprotected sex is the single biggest factor. Consistent condom use significantly reduces transmission, though it doesn’t eliminate risk entirely since condoms don’t cover all potentially infected tissue.
- Age: People under 25 account for a disproportionate share of infections, partly because of higher rates of new and concurrent partnerships and partly because the cervix in younger women is more biologically susceptible.
- Multiple partners: More partners means more potential exposures, particularly if testing between partners isn’t routine.
- Previous infection: Having had chlamydia before doesn’t protect you. Reinfection is common, especially if a partner wasn’t treated at the same time.
- No recent screening: Because most infections are silent, people who haven’t been tested recently may unknowingly carry and transmit the bacteria.
Testing and the Window Period
If you think you’ve been exposed, testing is straightforward. Most chlamydia tests use a urine sample or a swab and can detect the bacteria’s genetic material with high accuracy. The key detail is timing: testing too soon after exposure can produce a false negative. Most guidelines recommend waiting at least one to two weeks after a potential exposure before testing, since the bacteria needs time to multiply to detectable levels. Symptoms, when they occur, can take up to three weeks to appear, so a negative test combined with no symptoms at one week doesn’t guarantee you’re clear.
If your test comes back positive, treatment is simple and effective. A short course of antibiotics clears the infection in the vast majority of cases. Both you and any recent sexual partners need treatment to prevent passing the infection back and forth. Retesting about three months after treatment is recommended, since reinfection rates are high.
What Happens if It Goes Untreated
Left alone, chlamydia doesn’t resolve on its own, and the consequences become more serious over time. In women, the bacteria can travel from the cervix up 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. Ectopic pregnancy, where a fertilized egg implants outside the uterus, is also more likely after tubal damage from untreated chlamydia.
In men, untreated infection can cause inflammation of the tube that carries sperm from the testicle, leading to pain and, in rare cases, fertility problems. For both sexes, having an active chlamydia infection also makes it easier to acquire or transmit HIV if exposed.