What Are STIs? The Basics You Should Know

An STI (or STD — the terms are used pretty interchangeably) is an infection that is primarily transmitted through sexual contact. In Japanese, it's usually just called seibyō ("sexual disease"), but the more accurate umbrella term is "sexually transmitted infection."

This isn't something that only happens to "other people." According to WHO, more than one million new STIs are acquired worldwide every day. Anyone can be infected, regardless of age or gender.

Japan is no exception. Chlamydia, gonorrhea, genital herpes, and genital warts are tracked through "sentinel surveillance," meaning only around 1,000 designated clinics nationwide report cases — so the true national total is unknown. Syphilis is the one exception: it's a fully reportable disease, with 14,663 cases logged in 2024 (MHLW). Because so many STIs cause few or no symptoms, the real number of infections is almost certainly higher than what gets reported.

The key thing to remember: Bacterial STIs like chlamydia, syphilis, and gonorrhea are curable with the right treatment, especially when diagnosed early. Some viral infections — HPV, for instance — often clear up on their own. Herpes remains in the body for life, although symptoms can be managed. Left untreated, some STIs can lead to serious complications, including infertility, miscarriage, and organ damage.

Chlamydia

Chlamydia is one of the most commonly reported STIs in Japan. WHO calls it one of the most common bacterial STIs in the world.

The basics

• Reported cases: 29,795 in 2024 (provisional, sentinel-based)
• No symptoms: roughly 70–75% of women and 50% of men infected have no symptoms (ECDC and multiple peer-reviewed studies)
• Incubation: if symptoms do show up, it's usually within 1–3 weeks (CDC)
• Worth knowing: it spreads easily precisely because most people don't know they have it — and it can infect the throat and rectum, not just the genitals

If symptoms do show up

• Men: discomfort or mild pain while urinating, discharge, itching
• Women: unusual discharge, lower abdominal pain, bleeding between periods, discomfort urinating
• Throat infections: mild soreness, often mistaken for a cold — or no symptoms at all
• Rectal infections: discomfort, pain, or discharge

Left untreated

• Women: can lead to pelvic inflammatory disease (PID), damaging the fallopian tubes — a real risk factor for infertility and ectopic pregnancy
• Men: can cause epididymitis (inflammation of the epididymis)
• Also worth knowing: chlamydia is linked to a higher risk of HIV infection, and during pregnancy it's associated with preterm birth and low birth weight (WHO)

Read more: Why Is Chlamydia Called the "Hidden" STI?

Syphilis

Cases have been increasing since around 2011, and syphilis is now a significant public health concern (MHLW's syphilis Q&A; JIHS's detailed page).

The basics

• Reported cases: up from 5,871 in 2020 to 15,055 in 2023 (MHLW)
• Incubation: roughly 3 weeks to 3 months
• The tricky part: it moves through distinct stages, and symptoms can vanish on their own without the infection actually going away

How it progresses

Stage 1 (around 3 weeks after infection): A painless sore, known as a chancre, may appear where the infection entered the body — genitals, mouth, or anus. Important: it may heal on its own within a few weeks, but the infection is still there.

Stage 2 (3–12 months, if untreated): A faint reddish rash can spread across the body, including the palms and soles, sometimes with fever, fatigue, and swollen lymph nodes. Important: these symptoms can also fade on their own, which is exactly when people wrongly assume they're fine.

Latent syphilis (no symptoms at all): Even after stage 1 and 2 symptoms disappear, the bacteria are still present. The risk of sexual transmission is highest during the early stages of syphilis, particularly during the first year after infection. It's entirely possible to go years without realizing you're infected.

Late stage (years to decades later, if never treated): Can affect the brain, heart, and blood vessels (neurosyphilis, cardiovascular syphilis). This is rare today thanks to widely available antibiotic treatment (Japan's government public relations site).

During pregnancy: Untreated syphilis can pass to the fetus (congenital syphilis), potentially causing serious developmental problems, bone abnormalities, or hearing and vision loss (JIHS).

Read more: Why Are Syphilis Cases Rising in Japan?

Gonorrhea

The basics

• Reported cases: 8,789 in 2024 (provisional, sentinel-based)
• Symptoms: often silent, especially in women and in throat infections
• Growing concern: drug-resistant strains are on the rise worldwide (WHO)
• Can also infect the throat and rectum, not just the genitals

If symptoms do show up

• Men: sharp pain urinating, thick yellow or greenish discharge, swelling
• Women: discharge, lower abdominal pain, painful urination, bleeding between periods
• Throat/rectal infections: soreness or discomfort — frequently no symptoms at all

Left untreated

• Women: PID, which can lead to infertility
• Men: epididymitis
• Growing antibiotic resistance is making treatment increasingly difficult

Genital Herpes

The basics

• Global prevalence: more than one in five people aged 15–49 worldwide are estimated to have genital herpes, according to the Japan WHO Association, citing WHO estimates
• By sex: WHO's 2020 estimates put HSV-2 prevalence at 17.0% in women versus 9.7% in men. Transmission from men to women is more efficient than from women to men
• Symptoms: most infected people never notice anything
• Cause: herpes simplex virus, HSV-1 or HSV-2
• Incubation: typically a few days, though symptoms can take much longer to show up in some cases

When symptoms do appear

First outbreak (usually the worst one): painful blisters or sores, itching, painful urination, fever, fatigue, swollen lymph nodes — can last 1–2 weeks.

Later outbreaks: milder, and usually clear up within a week.

Worth knowing: the virus remains in the body for life — it lies dormant and tends to flare up when you're run down or stressed. And even when there's no visible outbreak, the virus can still shed from the skin, meaning it's possible to pass it on without realizing you're contagious.

Left untreated

• HIV risk: HSV-2 infection raises the risk of acquiring HIV (WHO)
• Newborn risk: a baby can be exposed during vaginal delivery if the mother is infected
• In immunocompromised people: symptoms can be more severe, occasionally leading to complications like meningoencephalitis or pneumonia

Genital Warts (Condyloma)

The basics

• Cause: low-risk HPV types 6 and 11 (Tokyo Metropolitan Infectious Disease Surveillance Center, JIHS)
• Incubation: a few weeks to 2–3 months

Symptoms: small pink or brown bumps — flat, raised, or cauliflower-like — around the genitals, cervix, vagina, or anus. Often itchy or uncomfortable, but plenty of people don't notice anything at all. Left alone, the warts can grow and multiply.

About the cancer connection

According to the Tokyo Metropolitan Infectious Disease Surveillance Center, the warts themselves are benign and often clear up naturally. That said, women infected with high-risk HPV types (16, 18, 52, 58, etc.) can develop cervical cancer from those specific types. In other words — and this is a common point of confusion — the low-risk HPV that causes visible warts (types 6 and 11) doesn't somehow turn into a cancer-causing high-risk type. What happens is that someone can be infected with both a low-risk and a high-risk type at the same time. CDC is clear that having genital warts is not, by itself, a reason to get cervical cancer screening more often than usual — women aged 21 to 65 should simply keep up with routine screening as recommended for their age group.

Left untreated

• Warts can grow larger and more painful, and spread to partners
• If a mother is infected during pregnancy, the baby can be exposed during delivery, occasionally leading to a respiratory infection in the newborn (Tokyo Metropolitan Infectious Disease Surveillance Center)

Mycoplasma genitalium

The basics

• Global prevalence: around 2% in low-risk women, but anywhere from 0% to 42% in high-risk populations (Deguchi et al., Emerging Infectious Diseases, 2015)
• Cause: the bacterium Mycoplasma genitalium
• Incubation: roughly 3 days to 5 weeks
• Worth knowing: it looks and behaves a lot like chlamydia, but drug resistance is a much bigger problem

Symptoms (similar to chlamydia)

• Men: urethritis — painful urination, discharge. Japanese studies attribute roughly 10–20% of non-gonococcal urethritis cases in men to this bacterium
• Women: cervicitis — unusual discharge, lower abdominal pain
• Symptoms range from nothing at all to fairly pronounced — and on symptoms alone, it's genuinely hard to tell this apart from chlamydia

Left untreated

• Studies have linked M. genitalium infection with a roughly twofold increased risk of cervicitis, pelvic inflammatory disease (PID), and preterm birth (CDC)
• Some studies have reported an association with tubal infertility, although the evidence is still limited
• Because M. genitalium can cause no symptoms in women, it may go undetected. Infection has been associated with complications such as pelvic inflammatory disease and infertility. M. genitalium infection during pregnancy has also been associated with an increased risk of preterm birth

The bigger issue: drug resistance

Drug-resistant Mycoplasma genitalium is a real and growing problem in Japan too. Multiple surveys conducted in Tokyo between 2019 and 2025 found that 89.9%–94.4% of samples carried mutations linked to macrolide resistance (i.e., resistance to azithromycin) — study 1, study 2. Resistance to fluoroquinolones (like levofloxacin and sitafloxacin) is climbing too, and one study found around 89% of strains were resistant to both drug classes — making treatment progressively harder.

Most STIs are quiet

Most sexually transmitted infections cause mild or no symptoms at all — which is exactly why they're so easy to miss. Around 70–75% of women and 50% of men with chlamydia have no symptoms whatsoever. That's one of the main reasons these infections go undetected for so long.

"The symptoms went away" doesn't mean "I'm cured"

Early symptoms can disappear on their own, but that doesn't mean the infection is gone. With syphilis specifically: if it isn't treated, the infection persists even after visible symptoms fade, and the first year of latent infection in particular still carries some risk of passing it on (JIHS). Symptoms vanishing is not the same thing as being cured.

The main route: sex

STIs are mostly passed on through vaginal, anal, and oral sex — any contact where infected fluids (semen, vaginal secretions, blood) reach mucous membranes or broken skin.

A few things worth keeping in mind:

1. Oral and anal sex aren't "safe" just because there's no condom involved. Plenty of people assume these don't carry STI risk — depending on the infection, they absolutely can.
2. A higher number of sexual partners can increase your chances of being exposed to an STI.
3. You can pass on an infection even when you have no symptoms.

Can you get one without having sex?

You won't get an STI from sitting on a public toilet seat or using a hot tub. There is no good evidence that these are meaningful routes of STI transmission — most STI pathogens simply don't survive long outside the body.

That said, some infections genuinely can spread outside of sex, through contact with fluids like semen, saliva, vaginal secretions, breast milk, or blood:

• Sharing razors or toothbrushes: if there's blood involved, this can — very rarely — spread HIV or hepatitis B/C (CDC, NIH)
• Sharing unwashed sex toys: a 2014 study out of Indiana University found HPV DNA on vibrators right after use — and even after standard cleaning, some materials still tested positive 24 hours later. So cleaning helps, but it isn't a guarantee. NHS also notes shared toys can spread trichomoniasis, and recommends washing them between uses or covering them with a fresh condom each time
• Blood transfusions or shared needles: in Japan, all donated blood is screened for HIV and hepatitis
• Tattoos and piercings: unsterilized equipment can transmit infections through blood (CDC)
• Contact with an open sore or lesion
• Kissing: oral herpes (HSV-1) in particular can spread this way — WHO notes HSV-1 spreads mainly through oral contact
• Mother to child: some infections can pass from mother to baby during pregnancy, birth, or breastfeeding (WHO)
• Damp fabric: LA County Public Health notes trichomoniasis can survive a few hours on a damp sponge or towel, and CDC says pubic lice die within 24–48 hours off the body but bedding or towels can, rarely, play a small role in spreading them

Bottom line: everyday contact, such as toilet seats or hot tubs, is not a meaningful STI risk. The actual risk depends on the infection and the type of contact involved, including sexual contact, contact with infectious lesions or bodily fluids, and — in some cases — sharing contaminated personal items.

Prevention

• Use condoms correctly and consistently — one of the most effective ways to reduce your risk of many STIs
• Fewer partners means less exposure
• Don't share razors, towels, or sex toys
• Get tested regularly, even without symptoms

Where to get tested

Clinics: Gynecology, dermatology, urology, and STI clinics can diagnose through a consultation, blood test, or swab. Tests ordered by a doctor may be covered by insurance.

Public health centers: Many offer free, anonymous testing, including evening/weekend hours and women-only testing days. You can search by location on MHLW's testing and counseling map.

At-home test kits: Depending on the test and where you live, some STIs can be checked using self-collection kits — you collect a urine or swab sample at home and send it to a laboratory. Chlamydia, gonorrhea, trichomoniasis, and Mycoplasma genitalium are commonly available this way; syphilis and HIV usually aren't included, so check exactly what's covered before you use one.

STIs can happen to anyone. Because so many cause no symptoms, it's entirely possible to have one — and pass it on — without knowing it. A few things matter most:

1. Test if you have any reason to, symptoms or not
2. Make sure you and your partner get tested and treated when recommended
3. Keep up prevention habits

Bacterial STIs like chlamydia, syphilis, and gonorrhea are highly treatable when diagnosed early. If something feels off, don't hesitate to see a doctor or visit a public health center.