More than 30 different bacteria, viruses, and parasites are known to spread through sexual contact. Of these, the World Health Organization identifies eight pathogens responsible for the greatest number of infections worldwide. These eight are the ones you’re most likely to encounter, get tested for, or hear about from a healthcare provider.
The 8 Most Common STIs
The WHO groups the most prevalent sexually transmitted pathogens into two categories: four that are curable and four that are not.
The curable four are:
- Chlamydia (bacterial)
- Gonorrhea (bacterial)
- Syphilis (bacterial)
- Trichomoniasis (parasitic)
All four can typically be cleared with a single round of antibiotics. The incurable four are all caused by viruses:
- Human papillomavirus (HPV)
- Herpes simplex virus (HSV)
- HIV
- Hepatitis B
“Incurable” doesn’t mean untreatable. Antiviral medications can manage herpes outbreaks, suppress HIV to undetectable levels, and slow liver damage from hepatitis B. HPV often clears on its own within a couple of years, though certain strains can cause genital warts or lead to cancer if they persist.
Beyond the Big 8
The eight major STIs get the most attention, but they’re not the complete list. Dozens of other organisms can spread sexually, and some are gaining clinical importance.
Mycoplasma genitalium (often called Mgen) is one of the most notable. It’s a bacterial infection that often causes no symptoms at all, which makes it easy to pass along unknowingly. When symptoms do appear, they typically include unusual discharge or a burning sensation when urinating. Left untreated in women, Mgen can lead to pelvic inflammatory disease, which may cause scarring in the fallopian tubes, ectopic pregnancy, or infertility. It may also be linked to preterm delivery. The infection is curable with antibiotics, but drug-resistant strains are becoming more common.
Other sexually transmitted organisms include hepatitis C, pubic lice, scabies, and certain strains of bacteria that cause conditions like lymphogranuloma venereum (a more aggressive form of chlamydia). Hepatitis A can also spread through sexual contact, particularly oral-anal routes.
STI vs. STD: What’s the Difference?
You’ll see both terms used interchangeably, but they mean slightly different things. An STI is a virus, bacterium, fungus, or parasite that someone picks up through sexual contact. An STD is what develops when that infection progresses to cause noticeable symptoms or health problems. In other words, every STD starts as an STI, but not every STI becomes an STD. Many infections, like chlamydia or HPV, can live in your body for months or years without causing any symptoms at all. The medical field has largely shifted toward using “STI” because it’s more accurate and reduces the stigma of the word “disease.”
How They Spread
Most people associate STIs with vaginal or anal intercourse, but transmission routes are broader than that. Oral sex carries real risk for gonorrhea, syphilis, herpes, and HPV. Even HIV can be transmitted orally if the infected person has a detectable viral load.
Some infections don’t require penetrative sex at all. Herpes, HPV, and syphilis can all spread through direct skin-to-skin contact with an infected area. This means condoms reduce risk significantly but don’t eliminate it entirely for these three, since the infection may be present on skin the condom doesn’t cover.
How Common STIs Actually Are
STIs are far more widespread than most people realize. In the United States alone, provisional 2024 data from the CDC recorded roughly 1.5 million chlamydia cases, 543,000 gonorrhea cases, and 190,000 syphilis cases. These are only the infections that were diagnosed and reported. The true numbers are higher because many infections go undetected.
Chlamydia is by far the most frequently reported, at about 446 cases per 100,000 people. Gonorrhea follows at 160 per 100,000, and syphilis at 56 per 100,000. These figures don’t include viral STIs like herpes and HPV, which are even more prevalent. Most sexually active adults will contract at least one strain of HPV at some point in their lives.
What Standard Testing Covers
There’s no single test that screens for every STI at once. What gets checked depends on your age, sex, sexual partners, and risk factors. A typical screening for a sexually active young woman includes chlamydia and gonorrhea. HIV screening is recommended at least once for all adults between 13 and 64. Syphilis testing is recommended for people at increased risk, including men who have sex with men and all pregnant women.
Some infections are notably absent from routine screening. Herpes, for instance, is not recommended as a standard blood test for people without symptoms because the results can be difficult to interpret and don’t change management in most cases. Mycoplasma genitalium, trichomoniasis, and HPV also require specific requests depending on your situation. Hepatitis B screening is recommended for people with multiple partners or other risk factors, while hepatitis C screening is recommended for all adults at least once.
If you’re concerned about a specific exposure, it helps to ask your provider exactly which infections are being tested rather than assuming a “full panel” covers everything. The answer varies from clinic to clinic, and knowing what was and wasn’t checked saves you from a false sense of security.