More than 30 distinct pathogens can spread through sexual contact, but eight are responsible for the vast majority of infections worldwide. Of those eight, four are curable with antibiotics and four are viral infections that stay in the body long-term. Understanding which infections fall into each category, how common they are, and how they differ gives you a practical framework for thinking about sexual health.
The Eight Most Common STIs
The World Health Organization groups the most significant sexually transmitted infections into two categories based on whether they can be cured.
The four curable infections are:
- Chlamydia, caused by bacteria, is the most frequently reported STI in the United States with over 1.5 million cases in 2024.
- Gonorrhea, also bacterial, accounted for about 543,000 reported U.S. cases in 2024.
- Syphilis, a bacterial infection that progresses through distinct stages if untreated, had roughly 190,000 total U.S. cases in 2024.
- Trichomoniasis, caused by a parasite rather than bacteria, is extremely common but often goes undiagnosed because routine STI panels don’t always include it.
The four incurable (but manageable) viral infections are:
- HPV (human papillomavirus), the single most widespread STI. More than 40 HPV types can infect genital areas, and 13 of those are classified as high-risk for causing cervical, throat, anal, and other cancers.
- Herpes (HSV), which comes in two forms. HSV-1 typically causes oral sores, and HSV-2 typically causes genital sores, though either type can appear at either location.
- HIV, which attacks the immune system and, without treatment, progresses to AIDS. Modern antiretroviral therapy can reduce the virus to undetectable levels.
- Hepatitis B, a virus that targets the liver. A highly effective vaccine exists, and most adults who contract it clear the virus on their own, though some develop chronic infection.
Beyond the Big Eight
Those eight infections account for most diagnoses, but the full list of sexually transmissible pathogens is considerably longer. Hepatitis C, while primarily spread through blood contact, can also transmit sexually, particularly among men who have sex with men. Mycoplasma genitalium is a bacterium only recently recognized as a significant STI. It causes symptoms similar to chlamydia and gonorrhea but requires different antibiotics, and many clinicians are still catching up on testing for it.
Lymphogranuloma venereum (LGV) is a more aggressive form of chlamydia that causes painful swelling in the lymph nodes. Once considered rare in high-income countries, it has re-emerged over the past two decades, with roughly 10,000 cases reported across 15 countries between 2004 and 2016. Pubic lice, scabies, and molluscum contagiosum round out the broader list. They spread through close skin contact during sex, though they can also spread through non-sexual contact.
Why “Curable” and “Incurable” Matter
The distinction between curable and incurable infections shapes what happens after a diagnosis. Chlamydia, gonorrhea, syphilis, and trichomoniasis are all treated with antibiotics, and once the infection clears, it’s gone. You can, however, catch any of them again. Gonorrhea in particular has developed growing resistance to several classes of antibiotics, which is why treatment recommendations have shifted over time.
The four viral infections work differently. Your body doesn’t fully eliminate HPV, herpes, HIV, or hepatitis B in the way antibiotics clear bacteria. HPV often resolves on its own within a year or two as the immune system suppresses it, but certain strains can persist and eventually cause cell changes that lead to cancer. Herpes stays dormant in nerve cells and can reactivate as outbreaks, though antiviral medication reduces both the frequency of outbreaks and the risk of passing the virus to a partner. HIV requires daily medication to keep the virus suppressed, but people on effective treatment live near-normal lifespans and cannot transmit the virus sexually when their viral load is undetectable.
Many Infections Cause No Symptoms
One of the most important things about STIs is that many of them produce no noticeable symptoms, especially in the early stages. Chlamydia is a textbook example: the majority of people who have it feel perfectly fine. The same is true for HPV, which most people carry at some point without ever knowing it. Gonorrhea can also be silent, particularly in women and when the infection is in the throat or rectum rather than the genitals.
Infections don’t have to be genital to matter. A CDC study of men who have sex with men found that 13.3% had chlamydia or gonorrhea at a rectal or throat site, and these infections are almost always asymptomatic. Because standard testing often focuses on urine or genital swabs, infections at other sites can go completely undetected unless a provider specifically tests for them. About one third of the men in that study hadn’t been tested for any STI in the past year.
How Testing Timelines Vary
Not every STI shows up on a test at the same time after exposure. Each infection has a “window period,” the gap between when you’re exposed and when a test can reliably detect it.
- HIV: A blood test using the antigen/antibody method catches most infections within 2 weeks and nearly all by 6 weeks. An oral swab takes longer, catching most by 1 month and nearly all by 3 months.
- Syphilis: A blood test catches most cases at 1 month and almost all by 3 months.
- Hepatitis B: Blood tests typically detect it at 3 to 6 weeks.
- Hepatitis C: Takes the longest. Blood tests catch most cases at 2 months, but full confidence requires waiting up to 6 months.
Chlamydia and gonorrhea have shorter window periods, generally detectable within 1 to 2 weeks after exposure via a urine or swab test. Testing too early after a potential exposure can produce a false negative, so timing matters.
U.S. Cases Are Declining, With One Exception
Provisional 2024 data from the CDC shows encouraging trends for the three nationally tracked STIs. The combined total of chlamydia, gonorrhea, and syphilis cases dropped 9% from 2023, marking a third consecutive year of decline. Chlamydia fell 8%, gonorrhea dropped 10%, and primary and secondary syphilis cases decreased 22%. Still, the raw numbers remain significant: over 2.2 million reported cases across just those three infections in a single year.
The notable exception is congenital syphilis, which occurs when a pregnant person passes syphilis to their baby during pregnancy or delivery. Cases have risen for 12 straight years, reaching nearly 4,000 in 2024. Congenital syphilis is entirely preventable with early prenatal screening and treatment, which makes this trend particularly frustrating from a public health standpoint.