There are dozens of sexually transmitted infections, caused by four different types of organisms: bacteria, viruses, parasites, and fungi. In the United States alone, more than 2.2 million cases of just three bacterial STIs (chlamydia, gonorrhea, and syphilis) were reported in 2024. That number doesn’t include viral infections like herpes and HPV, which affect millions more. Here’s a breakdown of the major STIs, what they do, and what sets each apart.
Bacterial STIs
Bacterial STIs are caused by bacteria and are generally curable with antibiotics. They’re also among the most common infections reported each year.
Chlamydia
Chlamydia is the most frequently reported STI in the U.S., with roughly 1.5 million cases in 2024. Most people who have it don’t know, because it often causes no symptoms at all. When symptoms do appear, they typically include burning during urination and unusual discharge. Left untreated, chlamydia can lead to pelvic inflammatory disease (PID) in women, which causes scar tissue in the reproductive tract and can result in infertility, chronic pelvic pain, or ectopic pregnancy. The more episodes of PID someone has, the higher the risk of permanent damage.
Gonorrhea
Gonorrhea infected over 543,000 people in the U.S. in 2024. Like chlamydia, many cases are asymptomatic, especially oral and rectal infections, where roughly 92% of people show no symptoms. When symptoms appear, they include discharge, burning during urination, and in women, bleeding between periods. Gonorrhea is treatable with antibiotics, though resistance to common treatments has been a growing concern for years. Untreated gonorrhea can also lead to PID and infertility.
Syphilis
Syphilis is less common than chlamydia or gonorrhea, with about 190,000 total cases reported in 2024, but it’s far more dangerous if left untreated because it progresses through distinct stages over years or even decades.
In the primary stage, a firm, round, painless sore appears where the bacteria entered the body. It lasts 3 to 6 weeks and heals on its own, which misleads many people into thinking it’s gone. The secondary stage brings a rough, reddish-brown rash that can show up on the palms of your hands or soles of your feet, along with fever, swollen lymph nodes, sore throat, patchy hair loss, and fatigue. After that comes the latent stage, a period with no visible signs that can last years. Without treatment, syphilis can eventually reach the tertiary stage, 10 to 30 years after the initial infection, damaging the heart, blood vessels, brain, and nervous system. Tertiary syphilis can be fatal. At any stage before that, antibiotics can cure it.
Mycoplasma Genitalium
Mycoplasma genitalium (often called Mgen) is a newer addition to the STI landscape. It causes symptoms similar to chlamydia, including urethritis in men and cervicitis in women, and is a suspected cause of some cases of PID. What makes Mgen particularly tricky is that resistance to the most common antibiotic used against it has reached 44% to 90% in the U.S. and other Western countries. Standard testing for it has only recently become available, and doctors often suspect it when a urethral or cervical infection keeps coming back after treatment for chlamydia and gonorrhea.
Viral STIs
Viral STIs cannot be cured, but most can be managed. The virus stays in your body, though treatments can reduce symptoms and lower the chance of passing the infection to others.
Genital Herpes (HSV)
Genital herpes is a chronic, lifelong infection caused by herpes simplex virus, most often type 2 (HSV-2). The majority of people infected with HSV-2 have never been diagnosed, because many have mild or unrecognized outbreaks. When symptoms do occur, they appear as painful blisters or ulcers in the genital area that come and go over time. Even between outbreaks, the virus can shed intermittently, meaning it’s possible to transmit herpes without visible sores. Daily antiviral medication can partially control symptoms and reduce viral shedding, but it doesn’t eliminate the virus from the body. Once the medication stops, the pattern of outbreaks may return.
Human Papillomavirus (HPV)
HPV is the most common STI overall. Most sexually active people will get at least one type of HPV at some point. There are many strains: some cause genital warts, while others cause no noticeable symptoms but can lead to cervical, throat, and anal cancers over time. The body’s immune system clears most HPV infections within a couple of years, but the high-risk strains that persist are responsible for nearly all cervical cancers. Vaccination before exposure is highly effective at preventing the most dangerous strains. For women, routine cervical screening (every 3 years starting at age 21, or every 5 years with combined HPV testing from age 30) catches precancerous changes early.
HIV
HIV attacks the immune system and, without treatment, progresses to AIDS. Early symptoms can feel like the flu, appearing weeks after infection, followed by a long period with no symptoms while the virus slowly damages immune function. Modern antiretroviral therapy can suppress HIV to undetectable levels, allowing people to live long, healthy lives and eliminating the risk of sexual transmission. The CDC recommends that all adults aged 13 to 64 get tested for HIV at least once.
Viral Hepatitis (B and C)
Hepatitis B and C both affect the liver and can be transmitted sexually, though hepatitis C spreads more efficiently through blood contact. Hepatitis B has a vaccine and is often cleared by the body, though some infections become chronic. Hepatitis C has no vaccine, but it’s now curable with antiviral treatment. All adults over 18 should be screened for hepatitis C at least once.
Parasitic STIs
Trichomoniasis
Trichomoniasis (trich) is caused by a parasite and is one of the most common curable STIs. About 70% of infected people have no symptoms. When symptoms do develop, typically within 5 to 28 days, women may notice itching, burning, genital redness, discomfort during urination, and a thin, fishy-smelling discharge that can be clear, white, yellowish, or greenish. Men may experience itching inside the penis, burning after urination or ejaculation, and penile discharge. Trich spreads through vaginal sex and can pass from penis to vagina, vagina to penis, or vagina to vagina. It’s cured with oral medication, which is also safe during pregnancy.
Pubic Lice and Scabies
Pubic lice (sometimes called crabs) and scabies are tiny parasites spread through close physical contact, including sex. Both cause intense itching. Pubic lice live in coarse body hair, while scabies burrow into the skin. Neither is dangerous, and both are treatable with topical medications.
Why Many STIs Go Undetected
The single most important thing to understand about STIs is that most of them can be completely invisible. Chlamydia and gonorrhea frequently cause no symptoms. About 61% of syphilis cases caught through screening are asymptomatic. The majority of people with genital herpes don’t know they have it. And 70% of trichomoniasis cases are silent. This means you can’t rely on symptoms to know whether you or a partner has an infection.
Regular screening is the only reliable way to catch these infections early. The CDC recommends annual chlamydia and gonorrhea testing for all sexually active women under 25, and for men who have sex with men. People at higher risk benefit from testing every 3 to 6 months. If you’re sexually active with new or multiple partners, routine testing is worth building into your healthcare, the same way you’d get a dental cleaning or an eye exam.
What Happens When STIs Go Untreated
The consequences of untreated STIs range from mild to severe, depending on the infection. Chlamydia and gonorrhea can silently cause pelvic inflammatory disease, which creates scar tissue and pockets of infection in the reproductive tract. PID can lead to chronic pelvic pain that lasts months or years, pain during sex or ovulation, ectopic pregnancy, and infertility. Delaying treatment significantly raises the risk of permanent damage.
Untreated syphilis can damage the brain, heart, and nervous system decades after the initial infection. Untreated HIV destroys the immune system. Persistent high-risk HPV strains can develop into cancer. Even trichomoniasis, though easily cured, can increase susceptibility to other STIs when left untreated. The common thread is that early detection and treatment prevent nearly all of these outcomes.