Why Is Chlamydia Called the "Hidden" STI? What You Actually Need to Know About the Risks—and What You Can Do

Chlamydia is described by PAHO, the WHO's regional office for the Americas, as "the most common bacterial sexually transmitted infection worldwide." According to the WHO Fact Sheet, an estimated 128.5 million new infections occurred worldwide among adults aged 15–49 in 2020, with an estimated prevalence of 4.0% among women and 2.5% among men.

Despite how widespread it is, chlamydia is one of those infections most people have heard of but don't actually know much about. The biggest reason is simple: most infections cause no noticeable symptoms at all. That lack of symptoms is exactly why chlamydia keeps spreading quietly.

Under Japan's Infectious Diseases Control Law, chlamydia isn't tracked the way syphilis is. Syphilis falls under "comprehensive reporting," meaning every diagnosed case is reported. Chlamydia, on the other hand, falls under "sentinel-site reporting"—only around 1,000 designated clinics nationwide report their case counts. So there's no exact national total; what we have is a trend based on those reports.

According to the Japan Institute for Health Security (formerly the National Institute of Infectious Diseases), which published its latest surveillance update in 2026, reported chlamydia cases from these sentinel sites had been climbing since 2016 and hit a record high in 2023. In 2024, the numbers dropped (Ministry of Health, Labour and Welfare, "Infectious Disease Surveillance").

Men's numbers swung more sharply than women's. By age, cases have consistently been highest among people in their 20s—especially early 20s—since the 2000s. At the same time, the agency flagged a rise among people in their late teens starting in 2023 as "a situation that warrants concern."

In other words, the recent dip in 2024 shouldn't be read as reassurance. The underlying pattern—rapid spread among young people—hasn't gone away.

How it spreads, and how long it takes to show up

Chlamydia spreads through vaginal, anal, and oral sex. According to the CDC's case definition, when symptoms do appear, they usually show up one to three weeks after exposure—but that's a big "if."

Why so many cases go undetected

That same CDC case definition puts the numbers plainly: about three-quarters of infected women (roughly 75%) and about half of infected men (roughly 50%) have no symptoms at all. Some people carry the infection for months or even years without ever knowing, which means they can unknowingly pass it on to partners.

Many people with chlamydia have no symptoms, or only mild ones. When symptoms do show up, they may not appear until as long as three weeks after having sex with someone who has chlamydia (WHO).

Common symptoms in men include a burning sensation when urinating, discharge from the penis, and pain in the testicles.

Common symptoms in women include a change in vaginal discharge, bleeding between periods or after sex, pain or discomfort in the lower abdomen, a burning sensation when urinating, and itching.

In both women and men, anal infection can cause pain, discharge, or bleeding around the anus. Oral infections are usually symptom-free, but when they do cause symptoms, they can look a lot like a cold—a scratchy or sore throat, phlegm, or coughing.

Left untreated, chlamydia can lead to some serious complications—something both WHO and CDC are clear about.

Risks for women

• Pelvic inflammatory disease (PID): CDC notes that in up to 40% of untreated women, the infection spreads to the uterus and fallopian tubes, causing PID.
• Infertility and ectopic pregnancy: if PID develops, scarring in the fallopian tubes can raise the risk of infertility and of ectopic pregnancy—a pregnancy that implants outside the uterus (WHO).

Risks for men

Complications in men are rarer, but the infection can spread to the epididymis (the tube that carries sperm), causing epididymitis (CDC).

Higher HIV risk

CDC data shows that women with chlamydia are up to five times more likely to contract HIV if exposed, compared with uninfected women.

Joint and eye complications

WHO notes that chlamydia can also trigger arthritis-like joint swelling and eye inflammation.

LGV (lymphogranuloma venereum)

Certain strains of chlamydia cause a more invasive, systemic infection called LGV. According to WHO, if it isn't treated early, LGV can lead to chronic oozing lesions around the anorectal area, strictures, or reactive joint pain and swelling.

Risks during pregnancy

WHO states that chlamydia during pregnancy is linked to preterm birth and low birth weight. Babies born to infected mothers can also develop conjunctivitis (an eye infection) or pneumonia from exposure during delivery.

Because chlamydia is so often symptom-free, regular testing is generally recommended for people in situations like these:

• You've had a new partner, have multiple partners, or your partner has other sexual partners (annual testing is recommended)
• You've had sex without a condom
• Your partner has been diagnosed with an STI
• You're pregnant or considering pregnancy
• You have symptoms

Given how common it is to have no symptoms, the takeaway isn't "no symptoms, no worries"—it's "if any of the above applies to you, get tested, symptoms or not."

Chlamydia is treatable with antibiotics. CDC's current guidelines recommend doxycycline (100 mg, twice a day, for 7 days) as the first-line treatment for non-pregnant patients, with a single dose of azithromycin sometimes used as an alternative. Completing treatment properly cures the infection in most cases.

But the fact that treatment works doesn't mean people actually get treated. Because symptoms are so often absent, many people never see a doctor in the first place, and the infection can quietly progress to PID, infertility, or other complications without them realizing it. Even where screening and treatment programs exist side by side, they haven't been enough to bring the epidemic under control—a gap both CDC and WHO acknowledge in their own materials. That gap—effective treatment, but a persistent epidemic—is arguably the biggest challenge in tackling chlamydia.

Basic precautions—condom use and regular testing—remain the most reliable tools available today. In practice, that means:

• Get tested even without symptoms, if anything on the checklist applies to you. Think of testing as a proactive way to put your mind at ease, not something you do only when something feels wrong.
• Get tested and treated together with your partner. If only one of you is treated, the other can pass the infection right back—so testing and treatment need to happen together.
• Use condoms correctly, every time. It's the most basic and effective prevention tool available.

The reason chlamydia earns the label "hidden" is precisely that it so rarely announces itself. Which means the only way to actually know your status isn't waiting for symptoms—it's the choice to get tested. Because it's so often silent, testing is what gives it meaning.