More than 30 different bacteria, viruses, and parasites can spread through sexual contact. Of these, eight cause the vast majority of infections: chlamydia, gonorrhea, syphilis, trichomoniasis, HIV, herpes (HSV), human papillomavirus (HPV), and hepatitis B. Four of those are curable with antibiotics. The other four are viral and stay in the body long-term, though all can be managed with treatment.
In 2024, the CDC counted more than 2.2 million reported cases of just chlamydia, gonorrhea, and syphilis in the United States. The actual number of people living with an STI is far higher, because many infections produce no symptoms and go undiagnosed.
Bacterial STIs
Bacterial STIs are the most common and the most treatable. All three major ones can be cured with antibiotics, but left untreated, they can cause serious long-term damage.
Chlamydia
Chlamydia is the most frequently reported STI in the U.S., with roughly 1.5 million cases in 2024. It is symptomless in an estimated 70% of women, which is why routine screening matters so much. When symptoms do appear, they typically include pain while urinating, unusual discharge, and lower abdominal discomfort. Men may notice a white or cloudy discharge from the penis and, less commonly, testicular pain.
Gonorrhea
Gonorrhea caused about 543,000 reported cases in 2024. Symptoms overlap heavily with chlamydia: burning during urination, unusual discharge (often green, white, or yellow), and pelvic pain. It can also infect the throat and rectum. Gonorrhea is curable, but antibiotic resistance has become a growing concern, making proper treatment important.
Syphilis
Syphilis progresses through distinct stages if untreated. The first sign is usually a small, painless sore at the site of infection. Weeks later, a second stage can bring a rash on the palms and soles of the feet, patchy hair loss, flu-like symptoms, and white patches in the mouth. If still untreated, syphilis can eventually damage the brain, heart, and other organs. About 190,000 total syphilis cases were reported in 2024, including nearly 4,000 cases of congenital syphilis, which passes from a pregnant person to the baby.
Viral STIs
Viral STIs cannot be cured, but most can be effectively managed. Some, like HPV, clear on their own in many people. Others, like HIV, require lifelong treatment.
Human Papillomavirus (HPV)
HPV is the most common STI overall. Most infections cause no symptoms and clear without treatment within a year or two. Certain strains cause genital warts, which can appear on and around the genitals, anus, and groin. Other high-risk strains can lead to cervical, anal, and throat cancers over time. Vaccination is highly effective at preventing the strains responsible for most cancers and warts. Cervical screening (Pap tests, often combined with HPV testing) is recommended every three to five years depending on age.
Herpes Simplex Virus (HSV)
Genital herpes is caused by HSV-1 or HSV-2. It produces recurring outbreaks of painful blisters or sores around the genitals or mouth. Many people carry the virus without ever having noticeable outbreaks. There is no cure, but antiviral medications reduce the frequency and severity of outbreaks and lower the risk of passing the virus to partners.
HIV
HIV attacks the immune system and, without treatment, progresses to AIDS. About 80% of people experience a short flu-like illness two to six weeks after infection, with fever, sore throat, body rash, swollen glands, and fatigue. After that initial phase, the virus can remain symptom-free for years while quietly damaging the immune system. Modern antiretroviral therapy allows most people with HIV to live long, healthy lives and reduce the virus to undetectable levels, which also prevents sexual transmission. Pre-exposure prophylaxis (PrEP) is available for people at higher risk and is highly effective at preventing infection.
Hepatitis B
Hepatitis B spreads through sexual contact, blood, and from parent to child during birth. Many adults clear the virus on their own and develop protective antibodies. Those who don’t clear it develop chronic hepatitis B, which can lead to serious liver disease over time. A functional cure (where the virus becomes permanently inactive) happens in fewer than 1% of chronic cases per year. Hepatitis B is preventable with a widely available vaccine. Hepatitis C, while more commonly spread through blood contact, can also transmit sexually and is now curable with antiviral treatment.
Parasitic STIs
Trichomoniasis
Trichomoniasis is caused by a microscopic parasite and is one of the most common STIs worldwide. Symptoms appear roughly 5 to 28 days after infection, though many people, especially men, never develop any. Women may notice a foul-smelling, frothy discharge that can be clear, white, yellow, or green, along with genital itching, burning, and pain during sex or urination. Men, when symptomatic, may have mild discharge, irritation inside the penis, and burning with urination. Trichomoniasis is easily cured with a single dose of antibiotics.
Pubic Lice and Scabies
These are ectoparasites, meaning they live on or in the skin rather than inside the body. Pubic lice (sometimes called crabs) attach to coarse body hair and cause intense itching. Scabies is caused by tiny mites that burrow into the skin, producing a rash and severe itching that worsens at night. Both spread through close skin-to-skin contact, including sexual contact, and both are treatable with topical medications.
Mycoplasma Genitalium: A Newer Concern
Mycoplasma genitalium was first identified in 1981 but was difficult to diagnose for decades because testing technology lagged behind. It is now recognized as more common than previously thought. Like chlamydia, it often causes no symptoms. When it does, the signs are similar: abnormal discharge, burning during urination, and pelvic or abdominal pain. Because its symptoms mimic other STIs, it is frequently misdiagnosed or missed entirely. Specific testing is now available, and treatment typically involves antibiotics, though resistance to standard options has been increasing.
Why Many STIs Go Unnoticed
One of the most important things to understand about STIs is that many produce no symptoms at all, sometimes for months or years. Chlamydia, gonorrhea, HPV, herpes, trichomoniasis, and HIV can all be carried and transmitted without the infected person knowing. This is why testing based on risk factors, not just symptoms, is the standard approach. Routine screening is recommended annually for sexually active people under 25, and more frequently for those with multiple partners or other risk factors.
What Happens if STIs Go Untreated
Untreated chlamydia and gonorrhea can cause pelvic inflammatory disease (PID) in women, a serious infection of the reproductive organs. PID creates scar tissue in and around the fallopian tubes, which can block them. One in eight women with a history of PID has difficulty getting pregnant. PID also raises the risk of ectopic pregnancy, where a fertilized egg implants outside the uterus, a potentially life-threatening situation. Long-term pelvic pain is another common consequence.
Untreated syphilis can damage the brain, nerves, eyes, and heart. Untreated HIV eventually destroys the immune system. HPV’s high-risk strains can develop into cancer. Even trichomoniasis, while less dangerous on its own, increases susceptibility to other infections, including HIV. Having any untreated STI generally makes it easier to acquire or transmit additional infections.
Testing Windows and Methods
Different STIs require different tests and have different windows, the minimum time after exposure before a test can accurately detect infection. Chlamydia and gonorrhea are typically detected through urine samples or swabs and can show up within one to two weeks after exposure. Syphilis, HIV, and hepatitis B require blood tests, and accurate results may take several weeks to a few months depending on the test type.
Herpes testing is more nuanced. Blood tests can identify HSV antibodies, but they are mainly useful in specific situations: when someone has recurring symptoms but negative swab results, when there is an unconfirmed clinical diagnosis, or when a partner has known genital herpes. HPV is not routinely tested for in men; in women, it is detected through cervical screening starting at age 21 or 25 depending on guidelines.
Retesting after treatment matters too. The CDC recommends retesting for chlamydia and gonorrhea about three months after treatment, because reinfection is common.