There are more than two dozen recognized sexually transmitted infections, caused by bacteria, viruses, parasites, or even fungi. They range from extremely common and easily curable (like chlamydia) to lifelong viral infections that require ongoing management (like HIV). Here’s a breakdown of the major ones, how they differ, and what you should know about each.
Bacterial STIs
Bacterial infections are the most commonly reported STIs in the United States, and the good news is they’re all curable with antibiotics. In many cases, a single dose is enough. The three that get tracked most closely are chlamydia, gonorrhea, and syphilis.
Chlamydia is the most frequently reported bacterial STI in the country, with over 1.5 million cases recorded in 2024 alone. It often causes no symptoms at all, which is why routine screening matters. When symptoms do appear, they typically involve unusual discharge or burning during urination. Left untreated, chlamydia can lead to pelvic inflammatory disease in women, which can cause chronic pain and fertility problems.
Gonorrhea came in at about 543,000 reported cases in 2024. It infects many of the same sites as chlamydia (genitals, rectum, throat) and can also be silent. Symptoms, when present, tend to be more noticeable: thick discharge and painful urination. One growing concern is antibiotic resistance, which has made some strains of gonorrhea harder to treat than they used to be.
Syphilis progresses through distinct stages if untreated. It starts as a painless sore (often unnoticed), then can produce a body rash, then goes dormant for years before potentially damaging the heart, brain, and other organs. The U.S. saw roughly 190,000 total syphilis cases in 2024, including nearly 4,000 cases of congenital syphilis, where the infection passes from a pregnant person to their baby. Syphilis is fully curable, especially in its early stages.
Two less commonly discussed bacterial STIs are Mycoplasma genitalium, which can cause urethritis and cervicitis and is increasingly recognized as a significant infection, and bacterial vaginosis, which disrupts the normal vaginal bacteria balance and is often associated with sexual activity though not always classified strictly as an STI.
Viral STIs
Viral STIs behave differently from bacterial ones. Most cannot be cured, but they can be managed, and some are preventable with vaccines.
Human papillomavirus (HPV) is the single most common STI. Most sexually active people will encounter at least one strain at some point. The majority of HPV infections clear on their own within a year or two. Certain high-risk strains, however, can cause cervical, throat, anal, and penile cancers. Other strains cause genital warts. The HPV vaccine (Gardasil 9) protects against nine strains, including the ones most likely to cause cancer and warts. It’s recommended starting at age 9 to 12, ideally before any sexual exposure. Kids who start before age 15 need only two doses; those who begin at 15 or older need three.
Genital herpes is caused by herpes simplex virus, usually type 2 (HSV-2) but increasingly type 1 (HSV-1, the same virus behind cold sores). Herpes causes recurring outbreaks of painful blisters or sores, though many people have mild or no symptoms and can still transmit the virus. There’s no cure, but daily antiviral medication reduces outbreaks and lowers the chance of spreading it to a partner.
HIV attacks the immune system and, without treatment, progresses to AIDS. Modern treatment has transformed HIV from a fatal diagnosis into a manageable chronic condition. People on effective treatment can reach an undetectable viral load, meaning they cannot transmit the virus sexually. HIV is also preventable with PrEP, a daily or on-demand medication taken before potential exposure.
Hepatitis B spreads through sexual contact, blood, and from parent to child during birth. Many adults who contract it recover fully on their own, but some develop chronic infection. Between 15% and 40% of people with chronic hepatitis B eventually develop serious liver disease, including cirrhosis or liver cancer. A highly effective vaccine exists and is part of the standard childhood immunization schedule. Hepatitis C, while more commonly spread through blood contact (like shared needles), can also be transmitted sexually, particularly among men who have sex with men. Unlike hepatitis B, hepatitis C is now curable with antiviral treatment.
Parasitic STIs
Trichomoniasis is caused by a microscopic parasite and is one of the most common curable STIs worldwide. It primarily affects the vagina and urethra. Symptoms can include itching, burning, unusual discharge, or discomfort during urination, but many people (especially those with penises) have no symptoms at all. A single course of oral antibiotics clears it.
Pubic lice (sometimes called “crabs”) are tiny insects that live in coarse body hair, primarily in the pubic area. The main symptom is intense itching. They’re treated with over-the-counter medicated washes applied to the affected area for about 10 minutes.
Scabies is caused by a microscopic mite that burrows into the skin, causing intense itching that often worsens at night. It spreads through prolonged skin-to-skin contact, including sexual contact. Treatment involves a prescription cream applied from the neck down, or an oral medication. Because the mite’s eggs survive the first treatment, a second round is typically needed about two weeks later.
Most STIs Have No Symptoms
One of the most important things to understand about STIs is that the majority don’t announce themselves. The World Health Organization estimates that over 1 million curable STIs are acquired every day worldwide, and most are asymptomatic. Chlamydia, gonorrhea, HPV, herpes, trichomoniasis, and even early-stage syphilis and HIV can all be present without causing any noticeable signs. This is precisely why STIs spread so effectively, and why testing based on risk factors rather than symptoms is the standard approach.
When Tests Become Accurate
If you’ve had a potential exposure, getting tested too soon can give you a false negative. Each infection has a different window before it shows up reliably on a test:
- Chlamydia and gonorrhea: Detectable within 1 week for most people, and nearly all cases show up by 2 weeks.
- Syphilis: Blood tests catch most infections at 1 month, but full confidence requires waiting 3 months.
- HIV: A blood-based antigen/antibody test picks up most cases within 2 weeks, catching nearly all by 6 weeks. An oral swab test takes longer: 1 month for most, 3 months for near-complete accuracy.
- Herpes: Antibody blood tests need about 1 month for initial detection, with 4 months for highest accuracy. Swab tests of active sores can work sooner.
- Trichomoniasis: Usually detectable within 1 week, with nearly all cases caught by 1 month.
- Hepatitis B: Blood tests typically work at 3 to 6 weeks.
- Hepatitis C: Antibody tests catch most cases at 2 months, but can take up to 6 months for full accuracy.
Prevention Beyond Condoms
Condoms remain the most widely available and effective barrier method for reducing STI transmission, but they aren’t the only tool anymore. The HPV and hepatitis B vaccines prevent infection entirely when given before exposure. PrEP prevents HIV. And a newer option called doxy-PEP uses a dose of the antibiotic doxycycline taken within 72 hours after sex to reduce the risk of chlamydia, gonorrhea, and syphilis. The CDC currently recommends discussing doxy-PEP with gay, bisexual, and other men who have sex with men, as well as transgender women, who have had at least one bacterial STI in the past year. Clinical trials have shown significant reductions in infection rates for those groups.
Regular screening, especially for people with new or multiple partners, remains the cornerstone of STI prevention. Since most infections produce no symptoms, routine testing is often the only way to catch and treat them before they cause complications or spread further.