Sexually transmitted diseases (also called sexually transmitted infections, or STIs) fall into three main categories: bacterial, viral, and parasitic. The most common ones in the United States are chlamydia, gonorrhea, syphilis, HPV, herpes, HIV, and trichomoniasis. In 2024 alone, the CDC recorded over 1.5 million cases of chlamydia, more than 543,000 cases of gonorrhea, and roughly 190,000 cases of syphilis. Many of these infections cause no symptoms at all, which is why regular testing matters even when you feel fine.
Chlamydia
Chlamydia is the most commonly reported STI in the country, with a rate of about 446 cases per 100,000 people. An estimated 77% of all chlamydia infections never produce symptoms. When symptoms do appear, they can include unusual discharge, burning during urination, and pelvic pain. Left untreated in women, chlamydia can lead to pelvic inflammatory disease, ectopic pregnancy, and infertility.
The infection is caused by bacteria and is curable with a short course of antibiotics. Because it so often flies under the radar, the CDC recommends annual chlamydia testing for all sexually active women under 25, and for older women with new or multiple partners.
Gonorrhea
Gonorrhea is the second most commonly reported bacterial STI, with over 543,000 cases reported in 2024. Like chlamydia, it frequently causes no symptoms. Roughly 45% of gonorrhea infections are entirely asymptomatic. When symptoms do show up, they typically include discharge from the penis or vagina, painful urination, and, in some cases, sore throat or rectal discomfort depending on the site of infection.
Gonorrhea is treatable, but drug resistance has become a growing concern, requiring higher doses of certain antibiotics than were once standard. Untreated gonorrhea can cause the same reproductive complications as chlamydia, including infertility. It can also spread to the bloodstream and joints.
Syphilis
Syphilis progresses through distinct stages, each with different signs. During the primary stage, a firm, round, painless sore appears at the site where the bacteria entered the body, usually on the genitals, anus, rectum, or mouth. Because the sore doesn’t hurt, many people never notice it. It heals on its own within three to six weeks, but the infection continues if untreated.
The secondary stage brings skin rashes, often on the palms of the hands or soles of the feet, along with flu-like symptoms: fever, swollen lymph nodes, sore throat, patchy hair loss, muscle aches, and fatigue. After this, syphilis enters a latent stage with no visible symptoms that can last years. Tertiary syphilis, which develops 10 to 30 years after the initial infection if never treated, can damage the heart, blood vessels, brain, and nervous system, and can be fatal. Syphilis can also spread to the brain at any stage, causing severe headaches, confusion, personality changes, and problems with memory and muscle movement.
Most people with untreated syphilis don’t develop tertiary disease, but the risk is serious enough that early treatment with antibiotics is important. Syphilis cases have been rising sharply in recent years, with about 190,000 total cases and nearly 4,000 cases of congenital syphilis (passed from mother to baby during pregnancy) reported in 2024.
HPV (Human Papillomavirus)
HPV is the most common viral STI overall. It encompasses many different strains: some cause genital warts, while others can lead to cervical, anal, throat, and other cancers. Most HPV infections clear on their own within a year or two without causing problems, but the high-risk strains that persist are responsible for nearly all cervical cancers.
Unlike bacterial STIs, HPV has no cure, though the body’s immune system clears most infections naturally. What does exist is a highly effective vaccine. The HPV vaccine is recommended at age 11 or 12 (and can start as early as 9), with catch-up vaccination available through age 26 for anyone who wasn’t vaccinated earlier. Kids who start the series before their 15th birthday need only two doses, spaced 6 to 12 months apart. Those who start at 15 or older need three doses. Some adults between 27 and 45 may also benefit from vaccination after discussing it with a clinician.
Genital Herpes (HSV)
Genital herpes is caused by herpes simplex virus, most often type 2 but increasingly type 1 as well. The hallmark symptom is periodic outbreaks of painful blisters or sores around the genitals, rectum, or mouth. Between outbreaks, the virus lies dormant in the nervous system, but it can still shed and spread to partners even when no sores are visible.
Herpes is a lifelong infection with no cure, but antiviral medications taken daily can reduce the frequency and severity of outbreaks and lower the risk of passing the virus to a partner. Many people with herpes have mild or infrequent outbreaks that become less bothersome over time. The infection is extremely common, and while the diagnosis can feel overwhelming, it rarely causes serious health problems in otherwise healthy adults.
HIV
HIV attacks the immune system and, without treatment, can progress to AIDS. Early symptoms often resemble the flu, appearing two to four weeks after infection, and then the virus can remain silent for years while continuing to damage immune function. Modern antiretroviral treatment has transformed HIV from a fatal diagnosis into a manageable chronic condition. People on effective treatment can live long, healthy lives.
One of the most important developments in HIV care is the concept that people with an undetectable viral load (meaning treatment has reduced the virus to unmeasurable levels in the blood) do not transmit HIV sexually. For prevention, PrEP is a medication taken before potential exposure that greatly reduces the chance of getting HIV. PrEP pills reach maximum protection for receptive anal sex after about 7 days of daily use, and for receptive vaginal sex or injection drug use after about 21 days.
The CDC recommends that everyone between 13 and 64 get tested for HIV at least once, with more frequent testing (every 3 to 6 months) for men who have sex with men and people who share injection drug equipment.
Trichomoniasis
Trichomoniasis is caused by a parasite, not a bacterium or virus. About 70% of infected people have no symptoms. When symptoms do appear, women may notice itching, burning, redness, discomfort while urinating, and a thin or greenish vaginal discharge with a fishy smell. Men may experience itching or irritation inside the penis, burning after urination or ejaculation, and penile discharge.
The parasite typically spreads from a penis to a vagina or vice versa, and can also pass between vaginas. It is curable with oral medication. Trichomoniasis is one of the most common STIs worldwide, yet it receives less attention than chlamydia or gonorrhea because it isn’t a nationally reportable infection in the same way.
Why Most STIs Go Unnoticed
The single most important thing to understand about STIs as a group is that the majority produce no symptoms, especially in their early stages. Chlamydia is asymptomatic in roughly three out of four cases. Gonorrhea is silent nearly half the time. Trichomoniasis shows no symptoms in 70% of infections. HPV and herpes can both be transmitted by people who have no idea they carry the virus. This means you cannot rely on feeling “normal” as evidence that you’re infection-free.
Who Should Get Tested, and How Often
Testing guidelines vary by age, sex, and risk factors. Sexually active women under 25 should be tested for chlamydia and gonorrhea every year. Women 25 and older with new or multiple partners should do the same. Sexually active gay and bisexual men should be tested for syphilis, chlamydia, and gonorrhea at least annually, with testing every 3 to 6 months if they have multiple or anonymous partners. Pregnant women should be tested for syphilis, HIV, hepatitis B, and hepatitis C early in pregnancy.
If you’ve had oral or anal sex, standard genital testing may not be enough. Throat and rectal tests can detect infections at those sites that a urine test or genital swab would miss entirely. Most STI tests are simple: a urine sample, a blood draw, or a swab, depending on the infection and the site being tested.