There are more than 20 sexually transmitted infections (STIs), and they fall into a few broad categories based on what causes them: bacteria, viruses, or parasites. Each type behaves differently in the body, produces different symptoms, and requires a different approach to treatment. In the U.S. alone, more than 2.2 million cases of just three STIs (chlamydia, gonorrhea, and syphilis) were reported in 2024.
Most STIs share one important trait: they’re often asymptomatic. Globally, the majority of curable STIs produce no noticeable symptoms at the time of infection, which is why routine screening matters more than waiting for something to feel wrong.
Bacterial STIs
Bacterial STIs are caused by bacteria and can be cured with antibiotics. The three most common are chlamydia, gonorrhea, and syphilis.
Chlamydia is the most frequently reported STI in the United States, with roughly 1.5 million cases in 2024. It usually causes no symptoms at all. When symptoms do appear, days to weeks after exposure, they can include unusual discharge, painful urination, or bleeding between periods. Because these overlap with urinary tract infections, chlamydia is easy to mistake for something else.
Gonorrhea is the second most common, with about 543,000 reported cases in 2024. It can infect the genitals, rectum, and throat, and like chlamydia, it frequently causes no symptoms, especially in women. When it does, discharge and burning during urination are the most typical signs. Antibiotic-resistant strains have become a growing concern, making timely treatment important.
Syphilis progresses through distinct stages if untreated. The first sign is typically a painless sore (called a chancre) at the site of infection. Left untreated, syphilis moves through secondary and latent stages and can eventually damage the heart, brain, and other organs. About 190,000 total syphilis cases were reported in 2024, including nearly 4,000 cases of congenital syphilis passed from mother to baby during pregnancy.
A lesser-known bacterial STI called Mycoplasma genitalium (Mgen) has gained attention in recent years. Like chlamydia, it often produces no symptoms, though some people notice discharge or burning during urination. It’s diagnosed through a urine sample or swab and treated with antibiotics.
Viral STIs
Viral STIs cannot be cured, but most can be managed effectively or prevented with vaccines. The major ones are HPV, herpes, HIV, and hepatitis B.
HPV (human papillomavirus) is the most common STI overall. Most sexually active people will contract at least one strain at some point. The majority of HPV infections clear on their own without causing problems, but certain strains can lead to genital warts, and others can cause cervical, throat, and anal cancers over time. A vaccine is available that prevents the most dangerous strains and is most effective when given before sexual activity begins.
Genital herpes is caused by herpes simplex virus (usually type 2, though type 1 can also cause it). It produces recurring outbreaks of painful blisters or sores on or around the genitals. Many people with herpes have mild or unrecognized symptoms and don’t know they carry the virus. Antiviral medications can reduce the frequency and severity of outbreaks and lower the risk of transmission, but the virus stays in the body permanently.
HIV attacks the immune system and, without treatment, progresses to AIDS. Modern antiretroviral therapy allows people with HIV to live long, healthy lives and reduce the virus to undetectable levels, at which point it effectively cannot be transmitted sexually. A preventive medication called PrEP is available for people at higher risk of exposure.
Hepatitis B is a viral liver infection that can be transmitted sexually, through blood, or from mother to child. The hepatitis B vaccine reduces the risk of infection by 80% to 100%. For those who do develop chronic hepatitis B, antiviral medications help limit liver damage. People living with HIV are especially encouraged to get vaccinated, since coinfection complicates treatment.
Parasitic STIs
Trichomoniasis (“trich”) is the most common curable STI worldwide. It’s caused by a microscopic parasite that typically spreads from penis to vagina or vagina to vagina during sex. Many people with trich have no symptoms. Those who do may experience itching, burning, unusual discharge, or discomfort during urination. It’s treated with oral medication, and both partners need treatment at the same time to prevent reinfection.
Pubic lice (sometimes called “crabs”) and scabies are also transmitted through close physical or sexual contact. Both cause intense itching and are treated with topical medications. They’re less medically serious than other STIs but easily spread in intimate settings.
Why Most STIs Go Unnoticed
The biggest misconception about STIs is that you’d know if you had one. In reality, the majority of infections produce no obvious symptoms, at least not initially. Chlamydia is a clear example: most people who have it feel perfectly fine. The same is true for HPV, early HIV, gonorrhea (particularly in women), and trichomoniasis.
This is exactly why screening guidelines don’t rely on symptoms. The U.S. Preventive Services Task Force recommends that all sexually active women age 24 and younger be screened annually for chlamydia and gonorrhea, along with women 25 and older who have risk factors like a new partner, multiple partners, or inconsistent condom use. Screening for men in the general population doesn’t yet have a formal recommendation, though men who have sex with men are typically advised to test more frequently.
What Happens If STIs Go Untreated
Untreated bacterial STIs can cause serious complications, particularly for women. Chlamydia and gonorrhea can both lead to pelvic inflammatory disease (PID), an infection of the uterus, fallopian tubes, or ovaries. One in eight women with a history of PID has difficulty getting pregnant. PID can also cause chronic pelvic pain, scar tissue in the fallopian tubes, and ectopic pregnancies, which are life-threatening. The longer treatment is delayed, the more likely these complications become.
Untreated syphilis progresses through increasingly dangerous stages over months and years. Untreated HIV gradually destroys the immune system. Chronic hepatitis B can lead to liver cirrhosis and cancer. Even HPV, which usually resolves on its own, can cause cancer if high-risk strains persist for years without detection.
Testing Windows for Accurate Results
If you’ve been exposed to an STI, testing too early can produce a false negative. Each infection has a “window period,” the time between exposure and when a test can reliably detect it.
- Chlamydia and gonorrhea: One week catches most cases; two weeks catches nearly all.
- Syphilis: One month catches most; three months catches nearly all.
- HIV (blood test): Two weeks catches most; six weeks catches nearly all.
- Herpes (blood test): One month catches most; four months catches nearly all.
- Trichomoniasis: One week catches most; one month catches nearly all.
- Hepatitis B: Three to six weeks.
- Hepatitis C: Two months catches most; six months catches nearly all.
- HPV: Three weeks to a few months, detected via Pap smear. There’s no general screening blood test for HPV.
Some infections, including genital warts, molluscum contagiosum, and pubic lice, have no screening test at all. They’re diagnosed visually when symptoms appear.