The most common STI in the United States is human papillomavirus, or HPV. More than 42 million Americans currently carry disease-causing strains, and roughly 13 million new infections occur every year. If you narrow the question to bacterial STIs (the kind that get reported to public health departments and treated with antibiotics), chlamydia takes the top spot by a wide margin.
HPV: The Most Widespread STI Overall
HPV is so common that nearly every sexually active person will contract at least one strain at some point in their life. Most of these infections clear on their own within a year or two without causing any noticeable problems. The strains that do cause disease fall into two broad categories: some cause genital warts, while others can lead to cancers of the cervix, throat, anus, and other areas over many years.
Because the immune system clears most HPV infections silently, there is no routine “HPV test” for the general population. Screening happens mainly through cervical cancer screening (Pap smears and HPV co-testing) in women. The most effective protection is the HPV vaccine, which covers the highest-risk strains and is recommended for preteens but can be given through age 26, and in some cases up to 45.
Chlamydia: The Most Reported STI
Among STIs that doctors are required to report to the CDC, chlamydia is far and away the leader. Provisional 2024 data shows a rate of 549.5 cases per 100,000 women and 335.3 per 100,000 men. For comparison, gonorrhea, the second most reported bacterial STI, comes in at 203.0 per 100,000 men and 115.8 per 100,000 women. Syphilis rates are lower still.
Those numbers almost certainly undercount the true burden. The reason is simple: chlamydia is notoriously silent. About 75 percent of women and 50 percent of men with chlamydia have no symptoms at all. People who feel fine don’t get tested, so their infections never enter the surveillance system.
Why Chlamydia Often Goes Unnoticed
When symptoms do appear, they typically show up one to three weeks after exposure. Women may notice unusual vaginal discharge, burning during urination, or bleeding between periods. Men are more likely to have discharge from the penis or a burning sensation when urinating. But for the majority of infected people, none of that happens, and the infection quietly persists.
This is what makes chlamydia particularly tricky. You can carry it for months or even years without knowing, passing it to partners the entire time. The lack of symptoms doesn’t mean the infection is harmless. It just means the damage can accumulate before anyone realizes there’s a problem.
What Happens if Chlamydia Goes Untreated
Left alone, chlamydia can cause serious, sometimes irreversible health problems. In women, the infection can spread upward into the uterus and fallopian tubes, causing pelvic inflammatory disease. That condition brings abdominal and pelvic pain and, over time, can lead to infertility or ectopic pregnancy (a pregnancy that implants outside the uterus, which is a medical emergency). Chlamydia also increases the risk of acquiring HIV.
In men, the infection can travel to the testicles, causing a painful condition called epididymitis. In rare cases, this too can affect fertility.
Pregnant women with untreated chlamydia face additional risks. The infection is associated with premature delivery and low birth weight. Babies born to infected mothers can develop eye infections or pneumonia shortly after birth.
Who Should Get Screened
Because so many infections are silent, screening is the main way chlamydia gets caught. The U.S. Preventive Services Task Force recommends annual chlamydia and gonorrhea screening for all sexually active women age 24 and younger, and for women 25 and older who have risk factors like a new partner or multiple partners. For men, there isn’t enough evidence yet for the task force to make a universal recommendation, though many clinicians still test men who have risk factors or symptoms.
There’s no set rule on exactly how often to rescreen after a negative result. The practical approach most guidelines suggest is retesting whenever your sexual history has changed since the last test, whether that means a new partner, inconsistent condom use, or a partner who tested positive.
How Chlamydia Is Treated
The good news is that chlamydia is completely curable with a short course of antibiotics. Treatment typically takes seven days. During that time, you should avoid sexual contact to prevent passing the infection to someone else. Sexual partners from the preceding 60 days also need treatment, even if they have no symptoms, to prevent reinfection.
Retesting about three months after treatment is recommended, because reinfection is common. This is often less about treatment failure and more about a partner who wasn’t treated or a new exposure.
How the Other Major STIs Compare
While chlamydia and HPV dominate in sheer numbers, the broader STI landscape in the U.S. includes several other infections worth knowing about:
- Gonorrhea is the second most reported bacterial STI, with rates roughly one-third to one-half of chlamydia’s. It can infect the same sites (genitals, throat, rectum) and carries similar risks for pelvic inflammatory disease and infertility if untreated. Growing antibiotic resistance makes gonorrhea a particular public health concern.
- Syphilis has much lower overall rates but has been climbing steeply in recent years, especially among men. Primary and secondary syphilis, the earliest and most infectious stages, occur at a rate of 17.6 per 100,000 men and 6.9 per 100,000 women. Rising rates of syphilis in women have also led to a troubling increase in congenital syphilis, which affects newborns.
- Herpes simplex virus (HSV) is extremely common but not nationally reportable, so exact case counts are harder to pin down. Like HPV, many carriers don’t know they’re infected.
The pattern across all of these infections is the same: many are asymptomatic, all are more manageable when caught early, and routine screening remains the most reliable way to catch them before they cause lasting harm.