There are more than 20 recognized sexually transmitted infections, caused by a wide range of bacteria, viruses, and parasites. The World Health Organization identifies eight of these as the most common and consequential, but the full list extends well beyond that core group. Understanding how these infections differ, which ones are curable, and which ones you can prevent with vaccines gives you a practical framework for protecting your health.
The 8 Most Common STIs
Eight pathogens account for the vast majority of sexually transmitted infections worldwide. They split evenly into two categories: four that are curable and four that are not.
The four curable STIs are all treated with antibiotics or antiparasitic medications:
- Chlamydia
- Gonorrhea
- Syphilis
- Trichomoniasis (caused by a parasite, not bacteria)
The four viral STIs cannot be fully eliminated from the body, though all of them can be managed:
- HIV
- Genital herpes (herpes simplex virus)
- HPV (human papillomavirus)
- Hepatitis B
Together, the four curable infections alone account for over one million new cases every single day globally. In the United States, provisional 2024 data from the CDC recorded roughly 1.5 million chlamydia cases, 543,000 gonorrhea cases, and 190,000 syphilis cases in a single year.
Beyond the Big 8
The total count of more than 20 STIs includes infections that get less attention but still affect millions of people. Pubic lice, genital warts (a specific manifestation of HPV), hepatitis C, and bacterial vaginosis can all be transmitted through sexual contact. Molluscum contagiosum, a viral skin infection, spreads easily through skin-to-skin contact during sex.
One infection gaining increasing recognition is Mycoplasma genitalium, a slow-growing bacterium that causes urethritis in men and has been linked to cervicitis, pelvic inflammatory disease, preterm delivery, and infertility in women. It is responsible for an estimated 15 to 20 percent of non-gonococcal urethritis cases in men and can be detected in 10 to 30 percent of women with cervical inflammation. Diagnosing it is difficult because the organism can take up to six months to grow in a lab culture, and resistance testing is not commercially available in the U.S. It is not part of routine screening, so many cases go undetected.
Curable vs. Manageable Infections
The distinction between curable and incurable STIs matters, but “incurable” does not mean untreatable. Antiviral medications for herpes and HIV can suppress the virus, reduce symptoms, and dramatically lower the chance of passing the infection to someone else. People with HIV who take daily medication and maintain an undetectable viral load have effectively zero risk of sexually transmitting the virus. Hepatitis B antivirals slow liver damage, and many people clear HPV on their own within one to two years without ever developing symptoms.
For the four curable infections, a course of antibiotics resolves the infection itself. The catch is that treatment cannot reverse damage already done. An untreated chlamydia or gonorrhea infection can progress to pelvic inflammatory disease (PID), which causes scarring in the fallopian tubes. One in eight women with a history of PID has difficulty getting pregnant. PID can also lead to ectopic pregnancy and chronic pelvic pain.
Why Most Infections Go Unnoticed
A major reason STIs spread so effectively is that the majority of infected people feel perfectly fine. Research pooling data from tens of thousands of participants found that roughly 61 percent of chlamydia infections, 53 percent of gonorrhea infections, and 57 percent of trichomoniasis infections in women produced no symptoms at all. Herpes and HPV are similarly quiet in most carriers. You cannot rely on symptoms to know whether you have an STI, which is why routine screening exists.
Who Should Get Tested, and How Often
Screening recommendations vary by age, sex, and risk level. Sexually active women under 25 should be screened annually for chlamydia and gonorrhea. Women 25 and older need screening if they have new or multiple partners or other risk factors. All pregnant women should be tested for syphilis at their first prenatal visit, and those at higher risk should be retested during the third trimester and at delivery.
Men who have sex with men face higher screening thresholds: annual testing for chlamydia, gonorrhea, syphilis, and HIV at minimum, with testing every three to six months if risk is elevated. All adults between 13 and 64 should be tested for HIV at least once. Hepatitis C screening is recommended for every adult over 18 at least once in their lifetime. Anyone treated for chlamydia, gonorrhea, or trichomoniasis should be retested about three months later, since reinfection is common.
Two STIs Are Vaccine-Preventable
Safe, highly effective vaccines exist for two of the four viral STIs: hepatitis B and HPV. The hepatitis B vaccine is part of the standard childhood immunization schedule in most countries and provides long-lasting protection. The HPV vaccine (Gardasil 9) protects against the strains responsible for most cases of cervical cancer, as well as cancers of the throat, anus, penis, vagina, and vulva, plus genital warts. The WHO has set a global target of reaching high HPV vaccination coverage by 2030 as part of its strategy to eliminate cervical cancer as a public health problem.
Both vaccines work best when given before a person becomes sexually active, but they still offer protection for many adults who did not receive them earlier. If you are unsure of your vaccination status, your healthcare provider can check antibody levels for hepatitis B or discuss whether the HPV vaccine is still appropriate for your age group.