Human papillomavirus, or HPV, is the most common sexually transmitted infection in the United States. More than 42 million Americans are currently infected with disease-causing types of HPV. When it comes to infections that are tracked through lab reporting, chlamydia holds the top spot, with roughly 1.5 million cases reported in 2024 alone. The broader picture is striking: about one in five people in the U.S. has a sexually transmitted infection on any given day.
HPV vs. Chlamydia: Why Two Answers Exist
The answer to “most common STD” depends on how you count. HPV wins by sheer volume of people infected at any one time. It spreads so easily through skin-to-skin contact that most sexually active people will get at least one type of HPV during their lives. Many never know it because the immune system clears most infections within a year or two without symptoms.
Chlamydia, on the other hand, is the most commonly reported STI because doctors and labs are legally required to notify public health authorities when someone tests positive. HPV doesn’t carry the same reporting requirement, so its true numbers come from population-level estimates rather than case-by-case tracking. That’s why you’ll see chlamydia called the “most common” in some sources and HPV in others. Both statements are correct, just measured differently.
Chlamydia by the Numbers
Provisional CDC data for 2024 recorded 1,515,985 chlamydia cases at a rate of about 446 per 100,000 people. That’s nearly three times the number of gonorrhea cases (543,409) reported in the same year. And those figures almost certainly undercount the real burden because so many infections go undiagnosed.
The reason for that gap is symptoms, or the lack of them. Roughly 75 percent of women and 50 percent of men with chlamydia have no symptoms at all. You can carry the infection for weeks or months, pass it to partners, and never realize anything is wrong. When symptoms do appear, they typically involve unusual discharge, burning during urination, or in women, bleeding between periods.
Left untreated, chlamydia poses a real risk. About 10 to 15 percent of women with untreated chlamydia develop pelvic inflammatory disease, an infection of the uterus and fallopian tubes that can cause chronic pain and infertility. In men, untreated infection can lead to painful swelling in the reproductive tract. The good news: chlamydia is curable with a short course of antibiotics, and testing is a simple urine sample or swab.
Other Common STIs in the U.S.
Beyond HPV and chlamydia, several other infections affect millions of Americans:
- Trichomoniasis: A parasite-caused infection with an estimated two million cases in the U.S. as of 2018. Like chlamydia, it often produces no symptoms and is easily treated. It’s not a reportable infection, so official case counts don’t capture its true reach.
- Gonorrhea: The second most commonly reported bacterial STI, with 543,409 cases in 2024. It can infect the genitals, throat, and rectum, and growing antibiotic resistance makes it a major public health concern.
- Syphilis: Cases increased 80 percent between 2018 and 2022, climbing from 115,000 to more than 207,000 and reaching the highest numbers since the 1950s. Particularly alarming is the rise in congenital syphilis, which passes from mother to baby during pregnancy.
The CDC’s prevalence estimates also include herpes (HSV-2), hepatitis B, and HIV, bringing the total picture to eight commonly tracked infections.
Who Is Most Affected
Young people bear a disproportionate share of the burden. Those between 15 and 24 account for half of all new STI cases, despite making up only 25 percent of the sexually active population. Biology plays a role here: the cervix in younger women is more susceptible to infection. But behavioral factors matter too, including less consistent condom use, more frequent partner changes, and lower rates of routine screening.
Screening guidelines reflect this reality. The U.S. Preventive Services Task Force recommends chlamydia and gonorrhea screening for all sexually active women age 24 and younger, and for women 25 and older who have risk factors like a new partner, multiple partners, a partner with an STI, or inconsistent condom use. For men, there isn’t enough evidence yet for a blanket screening recommendation, though testing is still appropriate based on individual risk.
Why So Many Cases Go Undetected
The biggest driver of STI spread is the silent nature of these infections. When three out of four women with chlamydia feel perfectly fine, and most HPV infections clear on their own without ever being noticed, people don’t seek testing. That creates a cycle: undiagnosed infections get passed to new partners, who also don’t know they’re infected.
Routine screening breaks that cycle. A chlamydia test takes minutes, costs little (and is covered by most insurance for recommended groups), and catches infections before they cause damage. HPV vaccination, ideally given between ages 9 and 12 but available through age 26 (and in some cases up to 45), prevents the strains most likely to cause genital warts and cancer. Together, testing and vaccination are the two most effective tools for reducing the overall burden of STIs in the U.S.