Sexually transmitted infections are extremely common. Globally, the World Health Organization estimated 374 million new infections in 2020 across just four types: chlamydia, gonorrhea, syphilis, and trichomoniasis. In the United States alone, more than 2.2 million cases of chlamydia, gonorrhea, and syphilis were reported in 2024. These numbers only reflect what gets diagnosed and reported, so the true figures are higher.
The Big Picture in the U.S.
The 2.2 million reported cases in 2024 cover only three bacterial infections that doctors are required to report to public health agencies. When you add in viral infections like HPV and herpes, plus underreported infections like trichomoniasis, the total number of people living with or newly acquiring an STI in any given year is dramatically larger.
To put the scale in perspective: the CDC estimates there were more than two million trichomoniasis infections in 2018 alone, and that’s a single infection that often flies under the radar because most cases produce no symptoms. Nearly everyone who is sexually active will get HPV at some point in their lives, according to the CDC. About 12% of Americans aged 14 to 49 are living with genital herpes (HSV-2), and nearly half carry oral herpes (HSV-1). These are lifelong viral infections, so the number of people currently carrying them only grows over time.
Which Infections Are Most Common
HPV is by far the most widespread. It’s so common that contracting it is considered a near-certainty for sexually active people. Most HPV infections clear on their own within a year or two, which is why many people never know they had one. The strains that cause health problems, like cervical cancer or genital warts, are a smaller subset, and vaccination has significantly reduced their prevalence among younger age groups.
Chlamydia is the most commonly reported bacterial STI. Globally, there were an estimated 129 million new chlamydia infections in 2020. It’s especially prevalent in younger adults and often causes no symptoms at all, which means many cases go undetected unless someone gets screened.
Trichomoniasis is surprisingly widespread, with 156 million estimated new cases globally in 2020, making it more common than gonorrhea or syphilis. It’s caused by a parasite rather than a bacterium or virus and is more common in women than men. It’s easily treated with antibiotics but rarely tested for unless symptoms appear.
Gonorrhea accounted for an estimated 82 million new infections globally in 2020. Syphilis, while less common at 7.1 million new cases, has been rising sharply in recent years and is particularly concerning because untreated syphilis can cause serious long-term damage to the heart, brain, and other organs.
Young Adults Carry a Disproportionate Share
People aged 15 to 24 account for half of all new STI cases, despite representing only about 25% of the sexually active population. Several factors drive this. Younger people are more likely to have new or multiple partners, less likely to use barrier protection consistently, and less likely to get tested regularly. Biologically, the cervix in younger women is also more susceptible to certain infections like chlamydia.
This concentration in younger age groups doesn’t mean older adults are in the clear. STI rates among people over 45 have been climbing for years, partly because this group tends to skip screening and may not consider themselves at risk.
Why the Real Numbers Are Higher Than Reports Suggest
Many STIs produce no noticeable symptoms, especially in the early stages. Chlamydia is often called a “silent” infection because the majority of people who have it feel perfectly fine. Gonorrhea frequently causes no symptoms in women and can be asymptomatic in men as well. Trichomoniasis goes unnoticed in most cases. Even genital herpes can be mild enough that people mistake outbreaks for skin irritation or ingrown hairs.
Because people without symptoms rarely seek testing, a large share of infections are never diagnosed. The 2.2 million reported U.S. cases represent only what clinicians caught and reported to health departments. The actual burden of infection in any given year is several times that number when you include undiagnosed and unreported cases across all STI types.
What This Means for Your Risk
If you’re sexually active, your lifetime odds of contracting at least one STI are high. This isn’t a scare tactic; it’s a reflection of how biology works. Many of these infections are easily treatable when caught early, and some, like HPV, your immune system will handle on its own in most cases. The real danger comes from infections that go undetected for months or years, because untreated chlamydia and gonorrhea can lead to infertility, and untreated syphilis can cause organ damage.
Routine screening is the most effective way to bridge the gap between how common STIs are and how often they get caught. Current guidelines recommend annual chlamydia and gonorrhea screening for sexually active women under 25, and regular testing for anyone with new or multiple partners. Herpes testing is not part of standard screening panels, which is one reason so many people carry HSV-2 without knowing it. If you want a complete picture of your status, you may need to specifically request certain tests.