Human papillomavirus (HPV) is the most common sexually transmitted infection in the world. About 85% of sexually active people will get an HPV infection at some point in their lifetime, and an estimated 300 million women worldwide are living with one at any given time. While HPV gets the top spot for overall prevalence, chlamydia is the most commonly reported bacterial STI, with over 1.5 million cases documented in the United States in 2024 alone.
The answer depends slightly on how you measure it. HPV dominates in total number of people infected, but because most cases clear on their own and aren’t formally reported, chlamydia tops the list of diagnosed and reported infections. Both deserve attention, and understanding the full landscape helps you protect yourself.
HPV: The Most Widespread STI Overall
HPV is so common that avoiding it entirely over a lifetime is nearly impossible if you’re sexually active. The virus spreads through skin-to-skin contact during vaginal, anal, or oral sex, and condoms reduce but don’t eliminate the risk because HPV can infect areas a condom doesn’t cover. Most people who contract it never know they have it. The immune system clears the majority of HPV infections within one to two years without causing any problems.
The concern with HPV is the small percentage of infections that don’t clear. Certain strains cause genital warts, while others, called high-risk types, can lead to cervical, throat, anal, and penile cancers over time. Cervical cancer is the most well-known consequence, which is why routine Pap smears and HPV tests are a standard part of women’s healthcare. The HPV vaccine is highly effective at preventing the strains responsible for most cancers and genital warts, and it’s recommended for preteens but can be given up through age 26 (and sometimes up to 45 after discussing it with a provider).
Chlamydia: The Most Reported Bacterial STI
Chlamydia is the STI you’re most likely to see show up on a test result. In 2024, the U.S. recorded roughly 1,516,000 reported cases, and the true number is certainly higher because many infections go undiagnosed. Women are disproportionately affected, with about 944,000 reported cases compared to 564,000 in men. The rate among women was 549.5 per 100,000 people, compared to 335.3 per 100,000 for men.
The reason for that gap is partly biological and partly about screening patterns. Women under 25 are routinely screened for chlamydia during gynecological visits, which catches infections that would otherwise go unnoticed. Men are less likely to be screened unless they have symptoms or specific risk factors.
Why Most People Don’t Know They Have It
Chlamydia’s biggest danger is its silence. About 75% of women and 50% of men with chlamydia have no symptoms at all. When symptoms do appear, they typically include unusual discharge, burning during urination, or pelvic pain in women. But the majority of infected people feel completely fine, which means the infection can persist for months and be passed to partners without anyone realizing.
Untreated chlamydia poses serious risks, especially for women. Research from the UK Health Security Agency found that roughly 26% of pelvic inflammatory disease (PID) cases within 12 months were attributable to untreated chlamydia infections. PID is an infection of the uterus, fallopian tubes, or ovaries that can cause chronic pelvic pain, ectopic pregnancies, and infertility. The good news is that chlamydia is completely curable with a short course of antibiotics, and early treatment prevents these complications.
Genital Herpes and the Full Picture
Genital herpes, caused by herpes simplex virus type 2, is another extremely common STI that often flies under the radar. Over 520 million people worldwide were estimated to be living with genital herpes as of 2020, making it the second most prevalent STI globally after HPV. Like HPV, many people with herpes have mild or no symptoms and don’t know they carry the virus.
Together, chlamydia, trichomoniasis, genital herpes, and HPV account for 98% of all prevalent STIs and 93% of all new infections. Trichomoniasis, a parasitic infection that’s easily treated but rarely tested for outside of women’s health visits, is another quiet contributor to that total. These four infections make up the vast majority of the STI burden, yet most people only think about gonorrhea, syphilis, or HIV when they hear “STI.”
Syphilis: A Growing Concern
While not the most common STI, syphilis deserves mention because of alarming recent trends. Cases of newborn syphilis, which occurs when a pregnant person passes the infection to their baby, have risen nearly 700% since 2015. Primary and secondary syphilis in adults did decline about 22% in 2024, marking the second consecutive year of decrease, but overall numbers remain far higher than they were a decade ago. Syphilis is curable with antibiotics, but left untreated it progresses through stages that can eventually damage the heart, brain, and other organs.
Who Should Get Tested and When
Because the most common STIs are often symptomless, screening is the only reliable way to catch them early. Current CDC recommendations call for annual chlamydia and gonorrhea screening for all sexually active women under 25, and for older women with risk factors like new or multiple partners. Men who have sex with men should be screened at least annually for chlamydia, gonorrhea, and syphilis, with more frequent testing (every 3 to 6 months) if they’re on PrEP, living with HIV, or have multiple partners.
Everyone between 13 and 64 should be tested for HIV at least once. Pregnant women have their own set of recommendations, including syphilis screening at the first prenatal visit and repeat testing in the third trimester if risk factors are present. HPV screening happens through Pap smears and HPV co-testing, typically starting at age 21 for cervical screening and age 25 for HPV-specific tests.
If you’re sexually active and haven’t been tested recently, a simple urine sample or swab can detect chlamydia and gonorrhea, while blood tests cover syphilis, HIV, and herpes. Many clinics offer bundled STI panels that check for multiple infections at once. Given that the most common STIs rarely announce themselves with symptoms, routine testing is the most practical thing you can do to stay ahead of them.