The most common STI in the United States is human papillomavirus, or HPV. About 85% of people will get an HPV infection at some point in their lifetime. Among bacterial STIs that are tracked through mandatory reporting, chlamydia ranks first by a wide margin, with over 1.5 million cases reported in 2024 alone.
The answer depends on how you count. HPV dominates in sheer number of people infected, but most cases clear on their own and go unnoticed. Chlamydia is the most commonly reported STI because every diagnosed case gets sent to public health authorities. Both matter for different reasons, and understanding the full landscape helps you know what to watch for and when to get tested.
HPV: The Most Widespread STI Overall
HPV is so common that it’s essentially a near-universal experience for sexually active people. With an 85% lifetime infection rate, most people will contract at least one strain of HPV whether they realize it or not. There are over 200 strains of the virus, and the vast majority cause no symptoms and resolve without treatment within a year or two as the immune system clears the infection.
The strains that do cause problems fall into two categories. Low-risk strains can cause genital warts, which are uncomfortable but not dangerous. High-risk strains, particularly types 16 and 18, can lead to cervical, throat, anal, and other cancers if the infection persists for years. This is why HPV screening is built into routine cervical cancer screening for women.
The HPV vaccine has dramatically changed this picture. Early vaccination results in a 40% reduction in cervical precancers and more than an 80% reduction in the overall risk of developing cervical cancer. The vaccine is recommended for preteens and is available through age 26 for most people, with some adults up to 45 eligible depending on their situation.
Chlamydia: The Most Reported Bacterial STI
Chlamydia accounted for roughly 1.52 million reported cases in the United States in 2024, making it the single most reported notifiable infection of any kind, not just among STIs. And that number almost certainly undercounts the real burden, because the majority of chlamydia infections produce no symptoms at all. Many people carry and spread the bacteria without ever knowing they’re infected.
When symptoms do appear, they typically show up one to three weeks after exposure. In women, that can mean unusual vaginal discharge, burning during urination, or bleeding between periods. In men, symptoms usually involve discharge from the penis, burning with urination, or pain and swelling in one testicle. But again, most infections are silent, which is exactly why routine screening matters so much.
Left untreated, chlamydia poses serious risks. About 10 to 15% of women with untreated chlamydia develop pelvic inflammatory disease, an infection of the uterus and fallopian tubes that can cause chronic pelvic pain and infertility. In men, untreated chlamydia can lead to infection of the tube that carries sperm, which is painful and can also affect fertility. The good news: chlamydia is curable with a short course of antibiotics, and treatment is straightforward when the infection is caught.
How Other Common STIs Compare
Gonorrhea is the second most reported bacterial STI, with about 543,000 cases in 2024. It often co-occurs with chlamydia and shares many of the same symptoms, including discharge and painful urination. Gonorrhea can also infect the throat and rectum, where it frequently causes no symptoms. Unlike chlamydia, gonorrhea has become increasingly resistant to antibiotics over the past two decades, which makes it a growing public health concern.
Trichomoniasis is caused by a parasite rather than a bacterium or virus, and it’s far more common than most people realize. National survey data from the CDC found a prevalence of about 2.1% among women and 0.5% among men aged 14 to 59. Like chlamydia, it often causes no symptoms, though some women experience itching, burning, or unusual discharge. Trichomoniasis is curable with a single dose of medication but is not part of standard STI screening panels, so it frequently goes undiagnosed.
Genital herpes caused by HSV-2 affects roughly 12% of Americans aged 14 to 49. When you include HSV-1 (traditionally associated with oral cold sores but increasingly a cause of genital herpes through oral sex), nearly half the population in that age range carries one form of the virus. Herpes is manageable but not curable. Many people with herpes have mild or no symptoms and don’t know they carry it.
Syphilis, while less common at about 190,000 total cases in 2024, has been rising sharply in recent years and is particularly dangerous because it progresses through stages. Untreated syphilis can eventually damage the brain, heart, and other organs. Congenital syphilis, passed from mother to baby during pregnancy, accounted for nearly 4,000 cases in 2024.
Why So Many Cases Go Undetected
The single biggest reason STIs spread so effectively is that most of them are silent. Chlamydia, gonorrhea, HPV, trichomoniasis, and herpes can all be present without any noticeable symptoms. A person can feel perfectly healthy, have no visible signs of infection, and still pass an STI to a partner. This is not an edge case. It’s the norm for many of these infections.
This is why screening guidelines exist independent of symptoms. The CDC recommends annual chlamydia and gonorrhea screening for all sexually active women under 25, and for older women with risk factors like new or multiple partners. Men who have sex with men are recommended to screen at least annually, and every three to six months if at higher risk. Screening recommendations are based on anatomy, so transgender men and gender-diverse people with a cervix follow the same guidelines as cisgender women.
The Global Picture
STIs are not just an American problem. The World Health Organization estimates that 374 million new infections of chlamydia, gonorrhea, syphilis, or trichomoniasis occurred worldwide in 2020, which works out to over one million new cases per day. Syphilis alone accounted for 8 million new infections globally in 2022, along with an estimated 700,000 cases of congenital syphilis. These numbers reflect a global challenge of limited access to testing, treatment, and prevention in many regions.
In the U.S., there has been some recent progress. The combined total of reported chlamydia, gonorrhea, and syphilis cases declined 9% from 2023 to 2024, marking a third consecutive year of decline. That still adds up to more than 2.2 million reported cases, so the burden remains enormous, but the trend is moving in the right direction.