Sexually transmitted infections are extraordinarily common. The CDC estimates that about one in five people in the U.S. had an STI on any given day in 2018, and globally, there are over one million new infections per day among adults aged 15 to 49. These numbers are far higher than most people expect, partly because the majority of infections cause no obvious symptoms and go undiagnosed.
STI Numbers in the United States
In 2024, more than 2.2 million cases of chlamydia, gonorrhea, and syphilis were reported to the CDC. That number, while enormous, only captures the infections that were actually diagnosed and reported. The true count is much higher because it excludes extremely common infections like HPV, herpes, and trichomoniasis, which aren’t tracked through standard reporting systems.
When the CDC ran a broader analysis including eight major STIs (chlamydia, gonorrhea, syphilis, trichomoniasis, herpes, HPV, hepatitis B, and HIV), they estimated roughly 68 million people in the U.S. were living with an STI at any given time. Chlamydia, trichomoniasis, genital herpes, and HPV alone accounted for 98% of all those existing infections and 93% of all new ones.
Global Numbers
The World Health Organization estimates 374 million new infections of just four curable STIs (chlamydia, gonorrhea, syphilis, and trichomoniasis) occurred worldwide in 2020. That works out to more than a million new cases every single day. Syphilis alone accounted for 8 million new infections in 2022, along with an estimated 700,000 cases of congenital syphilis, where the infection passes from mother to baby during pregnancy.
The Most Common Infections
HPV and herpes are by far the most widespread STIs because they persist in the body long-term and spread easily, often without symptoms. Nearly half the U.S. population aged 14 to 49 (47.8%) tested positive for HSV-1, the virus that causes oral herpes, in the 2015–2016 national survey. HSV-2, the type more closely linked to genital herpes, was present in 11.9% of people in that same age range. That prevalence climbs steeply with age: about 0.8% of teenagers carry HSV-2, compared to 21.2% of adults in their 40s. Women are roughly twice as likely as men to have it (15.9% vs. 8.2%).
HPV is even more common. Most sexually active people will encounter it at some point, and while the body clears most HPV infections on its own within a year or two, certain strains can lead to genital warts or cancers of the cervix, throat, and other areas. Vaccination has significantly reduced the prevalence of the most dangerous strains in younger age groups.
Among the reportable infections, chlamydia is the most frequently diagnosed, followed by gonorrhea and then syphilis. HIV saw 39,201 new diagnoses in the U.S. in 2023.
Young Adults Are Hit Hardest
People aged 15 to 24 accounted for 53% of all new STIs in 2020, despite making up a much smaller share of the sexually active population. Several factors drive this disparity. Younger people are less likely to use barrier protection consistently, may have more new sexual partners in a shorter period, and face bigger practical barriers to getting tested, including cost, stigma, and lack of access to confidential healthcare. Biologically, the cervix in younger women is also more susceptible to chlamydia and gonorrhea infection.
Why the Real Numbers Are Higher Than Reported
Official case counts dramatically undercount the true burden of STIs. The main reason is that most infections produce no symptoms at all. An estimated 77% of chlamydia cases and 45% of gonorrhea cases never cause noticeable symptoms. People who feel fine don’t get tested, and their infections never enter the surveillance system.
This matters beyond just statistics. Asymptomatic chlamydia and gonorrhea can still cause serious damage over time, including pelvic inflammatory disease and fertility problems, and people who don’t know they’re infected can unknowingly pass the infection to partners. It’s one reason routine screening is recommended for all sexually active women under 25 and for anyone with new or multiple partners.
Recent Trends
The combined total of reported chlamydia, gonorrhea, and syphilis cases in the U.S. declined 9% from 2023 to 2024, marking the third consecutive year of decline. That’s encouraging, but context matters: the numbers had surged in prior years, and 2.2 million reported cases is still a massive figure. Syphilis in particular had been rising sharply for over a decade before this recent dip, and congenital syphilis remains a serious concern.
HIV diagnoses have been gradually declining in some populations thanks to expanded testing and preventive medication, but progress has been uneven. Communities in the South and among Black and Latino men who have sex with men continue to see disproportionately high rates.
What These Numbers Mean for You
With one in five Americans carrying an STI at any given time, the odds of encountering one during your sexual life are high. Most of the common infections are either curable with a short course of antibiotics (chlamydia, gonorrhea, syphilis, trichomoniasis) or manageable with treatment (herpes, HIV). The infections that cause the most long-term harm tend to be the ones that go undetected for months or years.
Regular screening is the single most effective way to close the gap between reported and actual cases. If you’re sexually active, getting tested at least once a year for chlamydia and gonorrhea (more often with new partners) catches the infections most likely to be silently doing damage. HPV vaccination, available up to age 45, prevents the strains responsible for most HPV-related cancers. And consistent condom use reduces transmission risk for nearly every STI, though it’s more effective against some (like HIV and gonorrhea) than others (like herpes and HPV, which spread through skin contact beyond what a condom covers).