Eight sexually transmitted infections account for the vast majority of cases worldwide: chlamydia, gonorrhea, syphilis, trichomoniasis, herpes (HSV), human papillomavirus (HPV), hepatitis B, and HIV. Four of these are caused by bacteria or parasites and can be cured with antibiotics. The other four are viral infections that can be managed but not eliminated from the body. In the United States alone, more than 2.2 million cases of just chlamydia, gonorrhea, and syphilis were reported in 2024.
Chlamydia
Chlamydia is the most frequently reported STI in the United States, with roughly 1.5 million cases in 2024. It’s caused by bacteria and is fully curable with antibiotics. The catch is that it rarely announces itself. Early infections often cause few or no symptoms, which means many people pass it along without realizing they have it.
When symptoms do appear, they can include pain during urination, unusual discharge, or pelvic pain in women. Left untreated, chlamydia can lead to pelvic inflammatory disease, which can damage the reproductive organs and cause fertility problems. Screening is recommended annually for all sexually active women under 25 and for men who have sex with men. If you test positive and get treated, retesting about three months later is standard to make sure the infection hasn’t returned.
Gonorrhea
Gonorrhea is another bacterial infection, with about 543,000 reported U.S. cases in 2024. Cases have declined for three years in a row, down 10% from 2023, but it remains a serious concern for one major reason: growing antibiotic resistance. Between 2022 and 2024, resistance to the primary antibiotics used to treat gonorrhea rose sharply. Resistance to one key drug, cefixime, jumped from 1.7% to 11%, and resistance to ciprofloxacin now sits at 95%, making it essentially useless against the infection.
Symptoms often include thick, cloudy, or bloody discharge from the penis or vagina, painful urination, and in some cases, sore throat from oral transmission. But gonorrhea can also be asymptomatic, particularly in women. If untreated, it can spread to the blood or joints and, like chlamydia, can cause lasting reproductive damage. Screening guidelines mirror those for chlamydia: annual testing for sexually active women under 25 and for men who have sex with men, with more frequent testing for higher-risk groups.
Syphilis
Syphilis progresses through distinct stages, each with different symptoms, and can cause severe damage if left untreated for years. About 190,000 total syphilis cases were reported in the U.S. in 2024. While primary and secondary cases dropped 22% from the prior year, congenital syphilis (passed from mother to baby during pregnancy) increased for the twelfth consecutive year, reaching nearly 4,000 cases.
How Syphilis Progresses
In the primary stage, one or more firm, round, painless sores appear at the site where the bacteria entered the body, typically on the genitals, anus, rectum, or mouth. These sores last three to six weeks and heal on their own, which can trick people into thinking the problem has resolved. It hasn’t.
The secondary stage brings skin rashes, often on the palms of the hands or soles of the feet, that look rough and reddish-brown. The rash usually doesn’t itch and can be faint enough to miss. Fever, swollen glands, sore throat, patchy hair loss, and fatigue may accompany it. After this, syphilis enters a latent stage with no visible symptoms at all, sometimes for years.
Tertiary syphilis is rare but devastating. It develops 10 to 30 years after the initial infection and damages internal organs, potentially causing death. Syphilis can also affect the brain, eyes, and ears at any stage, leading to headaches, vision changes, hearing loss, and cognitive problems. The good news: syphilis is curable with antibiotics, especially when caught early. A simple blood test is all it takes to diagnose it.
Trichomoniasis
Trichomoniasis is the most common nonviral STI worldwide, caused by a parasite rather than bacteria. About 2.6 million people in the U.S. are affected, with prevalence notably higher in women (2.1%) than men (0.5%). It’s fully curable with a course of oral antibiotics.
In women, symptoms typically include vaginal discharge that can be clear, white, greenish, or yellowish, along with itching, burning, and discomfort during urination or sex. Most men with trichomoniasis have no symptoms. Because it often flies under the radar, many cases go undiagnosed and untreated.
Genital Herpes (HSV)
More than 520 million people worldwide live with genital herpes. Two strains of herpes simplex virus cause it. HSV-1 primarily spreads through oral contact and is traditionally associated with cold sores around the mouth, but it can also cause genital herpes through oral sex. HSV-2 spreads through sexual contact and is the more common cause of genital herpes.
Most people with herpes never know they have it. Symptoms are either absent or mild enough to go unnoticed. When outbreaks do occur, they involve small red bumps, blisters, or open sores around the genitals or anus (for HSV-2) or in and around the mouth (for HSV-1). HSV-2 is much more likely to cause recurring outbreaks than genital HSV-1.
Both strains can spread even when no sores are visible. Herpes is not curable, but antiviral medications reduce the frequency and severity of outbreaks and lower the risk of passing it to partners.
Human Papillomavirus (HPV)
HPV is the most common sexually transmitted infection overall. An estimated 300 million women worldwide have an HPV infection at any given time, and most sexually active people will encounter at least one strain during their lifetime. The vast majority of HPV infections cause no symptoms and clear on their own within a year or two.
When HPV does cause problems, the results depend on the strain. Some strains produce genital warts: small bumps in the genital area that can be flat, raised, or clustered in a cauliflower shape. Other high-risk strains cause no visible symptoms but can lead to cervical, throat, anal, and other cancers over time.
The HPV vaccine is the most effective prevention tool available. It’s recommended as part of routine vaccination at age 11 or 12, though it can be given as early as 9. Catch-up vaccination is recommended through age 26 for anyone not previously vaccinated, and adults 27 through 45 can discuss vaccination with their clinician if they missed it earlier.
HIV
HIV attacks the immune system and, without treatment, progresses to AIDS. Early infection may cause no symptoms at all, or it may produce flu-like symptoms a few weeks after exposure that resolve and don’t return for years. During that time, the virus is still active and transmittable.
HIV is not curable, but modern antiretroviral therapy can suppress the virus to undetectable levels, meaning a person on effective treatment cannot pass it to sexual partners. People with HIV who start treatment early and stay on it have near-normal life expectancies. Screening is recommended for all adults ages 13 to 64 at least once, with more frequent testing for those at higher risk. Pre-exposure prophylaxis (PrEP) is a daily or injectable medication that dramatically reduces the risk of getting HIV for people who are at increased risk.
Hepatitis B
Hepatitis B is a viral infection that targets the liver and spreads through sexual contact, shared needles, or from mother to child during birth. An estimated 254 million people worldwide live with chronic hepatitis B. Some people clear the virus on their own, while others develop a chronic infection that can lead to liver damage, cirrhosis, or liver cancer over decades.
Some people with hepatitis B never develop symptoms. When symptoms do occur, they can include fatigue, nausea, abdominal pain, and jaundice (yellowing of the skin and eyes). There is no cure for chronic hepatitis B, but antiviral treatment can control it. A highly effective vaccine exists and is part of the routine childhood immunization schedule.
Curable vs. Manageable
The simplest way to think about STIs is to split them into two groups. Chlamydia, gonorrhea, syphilis, and trichomoniasis are all curable with the right antibiotics. Herpes, HPV, HIV, and hepatitis B are viral and stay in the body, but all four have treatments that control symptoms or suppress the virus. Two of the four viral infections, HPV and hepatitis B, have vaccines that prevent infection altogether.
Because so many STIs produce no obvious symptoms, routine screening is the only reliable way to catch them. Testing recommendations vary by age, sex, and risk factors, but sexually active women under 25 and men who have sex with men generally benefit from annual screening for chlamydia and gonorrhea at a minimum. Anyone who is sexually active and has new or multiple partners benefits from periodic HIV and syphilis testing as well.