There are more than 20 sexually transmitted infections caused by bacteria, viruses, parasites, and even fungi. In the United States alone, more than 2.2 million cases of just three of them (chlamydia, gonorrhea, and syphilis) were reported in 2024. Here’s a full breakdown of every major STI, how they spread, and what they do to your body.
Bacterial STIs
Bacterial STIs are the most treatable category because antibiotics can cure them. But “curable” only matters if you catch them early. Left untreated, several of these cause permanent damage.
Chlamydia is the most commonly reported STI in the U.S., with over 1.5 million cases in 2024. It usually causes no symptoms at all. When symptoms do appear, they show up days to weeks after infection and often mimic a urinary tract infection: painful urination, unusual discharge, or bleeding between periods. Untreated chlamydia is a leading cause of pelvic inflammatory disease (PID), which can permanently damage the reproductive organs. More than 100,000 people in the U.S. become infertile from PID each year.
Gonorrhea accounted for over 543,000 reported cases in 2024. It infects the same areas as chlamydia (genitals, rectum, throat) and frequently occurs alongside it. Gonorrhea has become increasingly resistant to antibiotics over the decades, and the recommended treatment has shifted to a higher-dose injection to stay ahead of resistant strains. Untreated gonorrhea also leads to PID and infertility.
Syphilis progresses through distinct stages. It first appears as a painless sore (called a chancre) that heals on its own in three to six weeks, which tricks many people into thinking it’s gone. If untreated, a second stage follows with a rash, often on the palms and soles of the feet. Syphilis can eventually damage the brain, heart, and other organs if it advances to later stages. There were over 190,000 total syphilis cases reported in 2024, including nearly 4,000 cases of congenital syphilis passed from mother to baby during pregnancy.
Mycoplasma genitalium is a lesser-known bacterial STI that causes persistent urethritis in men and is linked to cervicitis, PID, preterm delivery, and infertility in women. It often doesn’t respond to the first antibiotic tried, making it a growing concern.
Bacterial vaginosis (BV) results from an imbalance in the normal mix of bacteria in the vagina. It’s not always classified as an STI because it can develop without sexual contact, but sexual activity increases the risk. BV causes fishy-smelling discharge and can increase vulnerability to other STIs.
Two rarer bacterial STIs still appear in parts of the world. Chancroid causes painful genital ulcers and is mostly found in tropical regions. Granuloma inguinale (also called donovanosis) produces painless, slowly expanding, beefy-red ulcers on the genitals. It’s extremely rare in the U.S. but still seen in parts of India, South Africa, and South America. Treatment requires weeks of antibiotics, and relapses can occur months after therapy ends.
Viral STIs
Viral STIs cannot be cured with antibiotics. Some can be managed with antiviral medications, and two have effective vaccines.
Human papillomavirus (HPV) is the single most common STI in the U.S. There are many strains: some cause genital warts, others cause no symptoms but can lead to cervical, throat, anal, and other cancers years later. HPV spreads through skin-to-skin contact, meaning condoms reduce but don’t eliminate the risk. A highly effective vaccine is available and is recommended before someone becomes sexually active.
Genital herpes is caused by herpes simplex virus, which has two types. HSV-1 traditionally causes oral cold sores but increasingly causes genital infections through oral sex. HSV-2 is the classic genital herpes strain. Both types cause recurring outbreaks of painful blisters or sores, though many people have mild or unrecognizable symptoms and unknowingly pass it on. Herpes spreads through skin-to-skin contact. Antiviral medications reduce the frequency and severity of outbreaks but don’t eliminate the virus, which stays in the body for life.
HIV attacks the immune system and, without treatment, progresses to AIDS. It spreads through blood, semen, rectal fluid, vaginal fluid, and breast milk. Modern antiviral therapy can suppress HIV to undetectable levels, meaning a person on effective treatment can live a normal lifespan and cannot transmit the virus sexually. There is no vaccine or cure.
Hepatitis B is a liver infection that spreads through blood, semen, vaginal fluids, rectal fluids, and saliva. Most adults clear the infection on their own, but some develop chronic hepatitis B, which can lead to liver damage or liver cancer over time. Antiviral medications slow liver damage in chronic cases. A safe, effective vaccine exists and is part of the routine childhood immunization schedule.
Hepatitis C spreads primarily through blood-to-blood contact and is less commonly transmitted through sex, though the risk increases with practices that involve blood exposure. Unlike hepatitis B, hepatitis C now has curative antiviral treatments that eliminate the virus in most people within 8 to 12 weeks. No vaccine is available.
Hepatitis A spreads through the fecal-oral route, which can happen during certain sexual activities. It causes acute liver inflammation but does not become chronic. A vaccine is available.
Mpox is a viral infection that has been spreading through sexual contact, particularly skin-to-skin contact during sex. It causes a distinctive rash with raised lesions, along with fever and body aches. A vaccine is available for people at higher risk.
Molluscum contagiosum is caused by a poxvirus and produces small, firm, dome-shaped bumps on the skin. It spreads through skin-to-skin contact, including sexual contact, and through shared towels or gym equipment. It’s generally harmless and resolves on its own, though the bumps can take months to clear.
Parasitic STIs
Trichomoniasis is the most common curable STI worldwide. It’s caused by a microscopic parasite and spreads through vaginal intercourse. Symptoms include itching, burning, and frothy, foul-smelling discharge, but many people (especially men) have no symptoms at all. It’s easily cured with a single course of medication.
Pubic lice (sometimes called “crabs”) and scabies are tiny parasites that spread through sexual contact or other close skin-to-skin contact. They can also spread through shared clothing, towels, or bedding. Both cause intense itching and are treated with topical medications.
Fungal STIs
Candidiasis, commonly known as a yeast infection, is caused by an overgrowth of Candida fungi. Vaginal yeast infections aren’t always sexually transmitted, as the yeast naturally lives in the body and can overgrow for many reasons. However, sexual contact can sometimes trigger or spread the infection. Symptoms include thick white discharge, itching, and irritation. Over-the-counter antifungal treatments are effective for most cases.
How Different STIs Spread
Not all STIs spread the same way, and understanding the difference matters for protection. Most STIs, including chlamydia, gonorrhea, HIV, hepatitis B, and trichomoniasis, spread through the exchange of bodily fluids during vaginal, anal, or oral sex. Condoms are highly effective at blocking these.
Others spread through skin-to-skin contact: herpes, HPV, molluscum contagiosum, syphilis (through contact with a sore or rash), pubic lice, and scabies. Because these infections live on skin surfaces rather than solely in fluids, condoms reduce the risk but don’t prevent transmission completely if the infected area isn’t covered.
Hepatitis C is primarily a blood-borne infection. Hepatitis A spreads through the fecal-oral route. HIV requires that the virus in blood or sexual fluids enters through broken skin or mucous membranes.
Why Many STIs Go Undetected
One of the most important things to know about STIs is that the majority cause no obvious symptoms, especially early on. Chlamydia is notorious for being “silent.” HPV almost never causes symptoms until it has already caused cell changes or warts. HIV’s early symptoms mimic a mild flu and then disappear for years. Syphilis’s first-stage sore is painless and heals on its own, making it easy to miss entirely.
This is why screening matters even when you feel fine. The U.S. Preventive Services Task Force recommends that all sexually active women age 24 and younger be screened annually for chlamydia and gonorrhea. Women 25 and older should be screened if they have risk factors like a new partner, multiple partners, or inconsistent condom use.
Testing Window Periods
If you’ve had a potential exposure, timing your test correctly makes the difference between an accurate result and a false negative. Each infection has a window period, the time between exposure and when a test can reliably detect it.
- Chlamydia and gonorrhea: Detectable at 1 week for most cases, 2 weeks catches nearly all
- Syphilis: Blood test catches most at 1 month, nearly all by 3 months
- HIV (blood test): Detectable at 2 weeks for most, 6 weeks catches nearly all
- HIV (oral swab): 1 month for most, 3 months catches nearly all
- Herpes: Blood antibody test catches most at 1 month, nearly all by 4 months
- Trichomoniasis: 1 week for most, 1 month catches nearly all
- Hepatitis B: 3 to 6 weeks
- Hepatitis C: 2 months for most, 6 months catches nearly all
- HPV: 3 weeks to a few months via Pap smear
Which STIs Are Curable and Which Are Not
Four of the most common STIs are fully curable with the right treatment: chlamydia, gonorrhea, syphilis, and trichomoniasis. Pubic lice, scabies, and granuloma inguinale are also curable. The key is catching them early, before they cause complications like PID or organ damage.
Four major viral STIs are not curable: HIV, herpes, hepatitis B, and HPV. All four can be managed. Antiviral medications keep HIV suppressed and herpes outbreaks infrequent. Hepatitis B antivirals slow liver damage. HPV infections often clear on their own within two years, though the strains that cause cancer require monitoring and sometimes treatment of precancerous changes. Hepatitis C, while viral, is now effectively curable in most people. Vaccines exist for HPV, hepatitis A, hepatitis B, and mpox, making prevention possible before exposure ever occurs.