Human papillomavirus, or HPV, is the most common sexually transmitted infection in the United States. More than 42 million Americans are currently infected with disease-causing types of HPV, and about 13 million new infections occur every year. If you’re counting only bacterial STIs, chlamydia takes the top spot, with roughly 1.5 million reported cases in 2024.
The distinction matters because HPV and chlamydia behave very differently, carry different risks, and require different responses. Here’s what you should know about each of them and the broader landscape of STIs in the U.S.
HPV: The Most Widespread STI Overall
HPV is so common that nearly every sexually active person will be exposed to it at some point. Most of the time, the immune system clears the virus on its own within a year or two, and the person never knows they had it. Between 70 and 90 percent of HPV infections produce no symptoms at all. That’s a big part of why it spreads so easily.
The concern with HPV isn’t the infection itself in most cases. It’s the small percentage of infections that persist and, over years or decades, lead to cancer. HPV is the primary cause of cervical cancer, and it also causes anal cancer, throat cancer, and cancers of the vulva and penis. Certain high-risk strains are responsible for virtually all cervical cancers worldwide.
The HPV vaccine is highly effective at preventing infection with the strains most likely to cause cancer and genital warts. It’s recommended for preteens and is available for people up to age 26 (and in some cases up to 45). There is no routine HPV screening test for men. For women, HPV testing is built into cervical cancer screening through Pap smears and HPV co-tests, typically starting at age 25.
Chlamydia: The Most Reported Bacterial STI
Chlamydia is a bacterial infection, which means it can be cured with antibiotics. In 2024, the CDC recorded about 1,516,000 reported chlamydia cases in the U.S., a rate of roughly 446 per 100,000 people. That makes it the single most reported notifiable infectious disease in the country, not just among STIs.
The problem with chlamydia is silence. Around 70 percent of women and 50 percent of men with chlamydia have no symptoms whatsoever. You can carry it for months without knowing, passing it to partners all the while. When symptoms do appear, they typically include unusual discharge, burning during urination, or pelvic pain in women.
Left untreated, chlamydia poses serious risks, especially for women. Roughly 1 in 10 women with untreated chlamydia will develop pelvic inflammatory disease (PID) within a year. Of those who develop PID, an estimated 11 percent will experience infertility, and about 8 percent will have an ectopic pregnancy, a potentially life-threatening condition where a fertilized egg implants outside the uterus. Men can also develop complications, including infection in the tube that carries sperm, though this is less common.
Treatment is straightforward: a week-long course of oral antibiotics clears the infection in most cases. A single-dose alternative is also available. The key challenge isn’t treatment. It’s getting tested in the first place.
Who Gets Tested and When
Because so many STIs are silent, screening guidelines exist to catch infections before they cause harm. The CDC recommends annual chlamydia and gonorrhea screening for all sexually active women under 25. Women 25 and older should be screened if they have risk factors like new or multiple partners. The same annual screening applies to men who have sex with men, with more frequent testing (every 3 to 6 months) recommended for those at higher risk, such as people on PrEP or those with HIV.
There are no routine chlamydia or gonorrhea screening recommendations for heterosexual men at low risk, though testing may be offered in high-prevalence settings like STI clinics or correctional facilities. Anyone with HIV should be screened at their first evaluation and at least annually after that.
Young people carry a disproportionate burden. People aged 15 to 24 account for half of all new STI cases despite making up only about 25 percent of the sexually active population. This age group is a particular priority for screening.
Other Common STIs to Know About
Gonorrhea often travels alongside chlamydia and shares many of the same characteristics: frequently asymptomatic (at least 50 percent of women and up to 40 percent of men show no symptoms), curable with antibiotics, and capable of causing serious reproductive harm if left untreated. Gonorrhea is less common than chlamydia but still accounts for hundreds of thousands of reported cases each year in the U.S.
Genital herpes, caused by herpes simplex virus type 2 (and increasingly type 1), affects an estimated 520 million people worldwide. About 70 percent of people with herpes don’t know they have it because outbreaks can be mild or absent entirely. Herpes is manageable with antiviral medication but not curable. It stays in the body for life, with the potential for periodic outbreaks that tend to become less frequent over time.
Syphilis has been rising sharply in recent years after decades of decline. While its total numbers remain well below chlamydia or gonorrhea, the rate of increase has alarmed public health officials. Syphilis progresses through stages and can cause devastating damage to the heart, brain, and other organs if untreated. It can also be passed from mother to child during pregnancy, a trend that has been climbing at an especially troubling pace.
Why Most STIs Go Undiagnosed
The single biggest reason STIs spread so widely is that most of them produce no obvious symptoms. Across the board, the asymptomatic rates are striking: 70 to 90 percent for HPV, around 70 percent for herpes, 50 to 70 percent for chlamydia (depending on sex), and up to half for gonorrhea. People who feel perfectly fine can unknowingly transmit infections for months or years.
Women face a particular disadvantage. Many STI symptoms in women, like mild pelvic discomfort or changes in discharge, overlap with normal variation or other common conditions. Women also face more serious long-term complications from undiagnosed infections, including infertility, chronic pelvic pain, and pregnancy complications. This is why screening guidelines are more aggressive for women, especially younger women.
Routine testing is the only reliable way to catch these infections early. If you’re sexually active with new or multiple partners, regular screening is more important than waiting for symptoms that may never come.