About one in five people in the United States has a sexually transmitted infection at any given time. That’s roughly 20% of the entire U.S. population, based on CDC estimates from 2018, the most comprehensive analysis available. The number covers eight common infections: chlamydia, gonorrhea, hepatitis B, genital herpes, HIV, HPV, syphilis, and trichomoniasis. Four of those, chlamydia, trichomoniasis, genital herpes, and HPV, account for 98% of all existing cases.
The Numbers Behind the Headline
That one-in-five figure represents prevalence, meaning the total number of people carrying an infection on any given day, whether or not they know it. Many of these infections, particularly HPV and genital herpes, are lifelong or long-lasting, which inflates the prevalence count compared to bacterial infections that can be cured with antibiotics. New infections each year carry an estimated $16 billion in direct lifetime medical costs, which gives some sense of how many people are being newly diagnosed or treated annually.
The reported numbers almost certainly undercount the real totals. Many STIs produce no symptoms at all, especially in the early stages. Chlamydia and gonorrhea frequently cause no noticeable problems, particularly in women. Trichomoniasis, the most common curable STI, often goes undetected for months or years. People who don’t feel sick don’t get tested, and infections that aren’t tested for don’t show up in surveillance data.
Genital Herpes and HPV Drive Most Cases
The two viral infections that persist in the body for years, genital herpes (HSV-2) and HPV, make up the bulk of the prevalence estimate. Among Americans aged 14 to 49, about 11.9% test positive for HSV-2, the type most associated with genital herpes. That prevalence climbs sharply with age: less than 1% of teenagers carry it, compared to over 21% of people in their 40s. Women are nearly twice as likely to be infected as men (15.9% vs. 8.2%).
HSV-1, traditionally thought of as oral herpes but increasingly a cause of genital infections too, is even more widespread at 47.8% of people aged 14 to 49. That rate has actually been declining over the past two decades, down from about 59% in 1999-2000. While that sounds like good news, lower childhood exposure to HSV-1 may leave more young adults vulnerable to acquiring it genitally later in life.
HPV is the single most common STI in the country. Most sexually active people will contract at least one strain at some point. Many HPV infections clear on their own within a year or two, but certain high-risk strains can lead to cervical, throat, and other cancers. Vaccination has been dramatically reducing infections among younger age groups since it became widely available.
Bacterial STIs: Treatable but Still Rising
Chlamydia, gonorrhea, and syphilis are the three bacterial STIs that doctors are required to report to public health authorities, which means we have the most precise case counts for them. In 2024, the combined total of reported chlamydia, gonorrhea, and syphilis cases declined 9% from the previous year, marking a third consecutive year of decreases. That’s a meaningful shift after years of steady increases.
Syphilis has been the most alarming trend in recent years. Total syphilis cases in 2024 reached 190,242, and primary and secondary syphilis (the most infectious stages) dropped 22% from 2023, which is the second year of decline. The bigger concern is congenital syphilis, when a pregnant person passes the infection to their baby. Those cases climbed for the 12th straight year, reaching nearly 4,000 in 2024. Congenital syphilis can cause stillbirth, severe birth defects, and infant death, making it one of the most urgent STI-related public health problems in the country.
HIV in the U.S. Today
As of 2023, about 1.13 million people aged 13 and older were living with diagnosed HIV in the United States. That year saw 39,201 new HIV diagnoses. It’s worth noting that a new diagnosis doesn’t always mean a new infection, since some people live with HIV for years before being tested. Modern treatment allows most people with HIV to live long, healthy lives and to reduce the virus to undetectable levels, which effectively prevents sexual transmission.
Who Is Most Affected
STIs don’t spread evenly across the population. Young people between 15 and 24 consistently account for a disproportionate share of new chlamydia and gonorrhea cases, partly because of biology (the cervix is more susceptible to infection in younger women) and partly because of lower rates of consistent condom use and screening. Women in this age group are recommended for routine chlamydia and gonorrhea screening every year.
Racial and ethnic disparities are stark. For HSV-2, prevalence among non-Hispanic Black Americans is 34.6%, compared to 3.8% among non-Hispanic Asian Americans. These gaps reflect longstanding inequities in healthcare access, poverty, and the concentration of infections within sexual networks rather than differences in sexual behavior. Similar disparities show up in syphilis, gonorrhea, and HIV data.
Men who have sex with men face elevated rates of several infections, including syphilis, gonorrhea, and HIV. Current guidelines recommend that sexually active men in this group be screened for chlamydia, gonorrhea, syphilis, and HIV at least once a year, and every 3 to 6 months if they have multiple partners or are on HIV prevention medication.
Why So Many Cases Go Uncounted
The official surveillance numbers capture only a fraction of actual infections. Chlamydia and gonorrhea must be reported when diagnosed, but millions of cases are never diagnosed in the first place because the person never experienced symptoms and never got tested. Herpes and HPV are not routinely tested for in standard STI panels, so most people carrying those viruses have no idea. The CDC does not recommend routine HSV-2 blood testing for people without symptoms because of the high rate of false positives and the psychological burden of a diagnosis that may never cause problems.
Screening guidelines aim to catch infections where early treatment makes the biggest difference. All adults and adolescents aged 13 to 64 are recommended for at least one HIV test. All adults over 18 should be screened for hepatitis C at least once. Beyond that, routine screening depends on your age, sex, sexual partners, and risk factors. If you’re a sexually active woman under 25, annual chlamydia and gonorrhea testing is standard. If you’re over 25 with a new partner or multiple partners, the same applies.
The Cost of Untreated Infections
New STI infections acquired in 2018 alone were estimated to carry nearly $16 billion in direct lifetime medical costs. That figure includes treatment for the infections themselves, management of complications like pelvic inflammatory disease and infertility from untreated chlamydia or gonorrhea, cancer treatment linked to HPV, and the lifetime cost of managing HIV. Most of this spending is concentrated in a small number of infections: HIV treatment accounts for the largest share, despite HIV being far less common than HPV or herpes, because antiretroviral therapy is expensive and lifelong.
The financial picture underscores why prevention and early detection matter so much. Bacterial infections caught early cost very little to treat. Left alone, they can cause permanent damage that costs orders of magnitude more to manage.