About 20 percent of the U.S. population, roughly one in five people, has a sexually transmitted infection at any given time. That estimate comes from CDC surveillance data covering eight common infections: chlamydia, gonorrhea, hepatitis B, herpes (HSV-2), HIV, HPV, syphilis, and trichomoniasis. Globally, the numbers are equally striking, with over 1 million new cases of just four curable STIs occurring every day.
How Common STIs Are in the U.S.
The one-in-five figure represents a snapshot of active infections on any given day in 2018, the most recent year with comprehensive prevalence data. That translates to roughly 68 million people carrying at least one STI at the same time. On top of that existing pool, about 20 million new infections occur each year.
Young adults carry a disproportionate share. People aged 15 to 24 make up just over 25 percent of the sexually active population but account for half of all new infections annually. This isn’t because younger people are necessarily riskier in their behavior. It reflects biology (cervical tissue in younger women is more susceptible to certain infections), lower rates of routine screening, and less accumulated immunity.
The Global Picture
Worldwide, the World Health Organization estimated 374 million new infections of chlamydia, gonorrhea, syphilis, and trichomoniasis among adults aged 15 to 49 in 2020. Those are just the four most common curable STIs and don’t include viral infections like HPV, herpes, or HIV. Syphilis alone accounted for 8 million new infections in 2022, a number that has been climbing steadily.
Prevalence by Infection
Not all STIs are equally common, and some are so widespread that framing them as rare or unusual misrepresents reality.
HPV is by far the most prevalent. Nearly everyone who is sexually active and unvaccinated will contract at least one strain of HPV at some point. Most of these infections clear on their own within a year or two without causing symptoms, but certain strains can lead to genital warts or cancers of the cervix, throat, and other areas. The HPV vaccine has dramatically reduced infection rates in vaccinated populations.
Herpes (HSV-2) affects about 16 percent of Americans aged 14 to 49, or roughly one in six people in that age range. That figure only covers HSV-2, the strain most associated with genital herpes. If you include HSV-1 (traditionally oral herpes, but increasingly a cause of genital infections too), the combined prevalence is far higher. Most people with herpes never receive a formal diagnosis because their symptoms are mild or absent.
Trichomoniasis is the most common curable STI in the world, affecting an estimated 2.6 million people in the U.S. at any given time. Population-level prevalence sits at about 2.1 percent among women and 0.5 percent among men. It’s caused by a parasite rather than a virus or bacterium and clears easily with a single course of antibiotics, but it often goes undetected because routine STI panels don’t always include it.
Chlamydia and gonorrhea are the bacterial infections most people think of when they hear “STD.” Both are curable with antibiotics, but they frequently produce no symptoms at all. Oral and rectal infections are asymptomatic more than 90 percent of the time, while genital infections are symptom-free in roughly 25 percent of cases. That silent nature is a major reason these infections spread so efficiently.
HIV is far less common in absolute numbers but carries the most significant long-term health consequences if untreated. Over 1.1 million people aged 13 and older were living with diagnosed HIV in the U.S. in 2023. An additional number remain undiagnosed, though updated estimates for that gap are still pending from the CDC.
Syphilis Is Surging
Syphilis deserves special attention because its trajectory has shifted dramatically. After decades of decline, cases have been rising sharply in the U.S. Maternal syphilis, which can cause stillbirth, severe birth defects, and infant death, increased 28 percent between 2022 and 2024. The rate climbed from about 280 cases per 100,000 births in 2022 to nearly 358 per 100,000 in 2024.
The increase cuts across all racial and age groups but hits some communities harder. American Indian and Alaska Native women experienced rates above 2,100 per 100,000 births in 2024, roughly six times the national average. Black women and Native Hawaiian or Pacific Islander women also face rates two to three times higher than the overall population. These disparities reflect gaps in prenatal care access and STI screening rather than differences in sexual behavior.
Why So Many Cases Go Undetected
A significant portion of people with an STI don’t know they have one. Chlamydia, gonorrhea, trichomoniasis, HPV, and even herpes can all be present without producing noticeable symptoms for months or years. This means the true prevalence of STIs is almost certainly higher than reported case counts suggest, since surveillance data only captures infections that get tested and diagnosed.
Screening habits play a large role. Many people only get tested when symptoms appear, which misses the majority of asymptomatic infections. Routine screening recommendations exist for sexually active women under 25 (annual chlamydia and gonorrhea testing), all pregnant women (syphilis, HIV, hepatitis B), and men who have sex with men (broader panels at least annually). But adherence to those recommendations is inconsistent, and many primary care visits skip the conversation about sexual health entirely.
What the Numbers Actually Mean
The one-in-five statistic can sound alarming, but context matters. The overwhelming majority of that 20 percent is made up of HPV and herpes infections, most of which never cause serious health problems. HPV clears on its own in most cases, and herpes, while lifelong, is manageable and often produces infrequent or no symptoms after the initial outbreak.
The infections that do cause serious complications, like untreated chlamydia leading to infertility, syphilis causing organ damage, or HIV progressing to AIDS, are all either curable or highly treatable when caught early. The gap between how common STIs are and how often they’re detected is the real public health problem. Regular screening, especially if you’re under 25, have new partners, or are pregnant, is the most effective way to close that gap.