There are more than 20 types of sexually transmitted infections (STIs), and they fall into three main categories: bacterial, viral, and parasitic. The most common ones include chlamydia, gonorrhea, syphilis, HPV, genital herpes, HIV, and trichomoniasis. Some are curable with antibiotics, while others stay in the body long-term but can be managed effectively. Here’s what you should know about each.
Bacterial STIs: Chlamydia, Gonorrhea, and Syphilis
Bacterial STIs are the most commonly reported infections and are all curable with antibiotics. In the United States alone, provisional 2024 data from the CDC recorded roughly 1.5 million chlamydia cases, 543,000 gonorrhea cases, and 190,000 syphilis cases.
Chlamydia is the single most reported STI. 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, about 10 to 15 percent of women with chlamydia develop pelvic inflammatory disease (PID), which can permanently damage the fallopian tubes and lead to infertility. Chlamydia can also cause “silent” infections in the upper reproductive tract, meaning damage accumulates without any noticeable signs.
Gonorrhea shares many of the same characteristics. It infects the same areas (cervix, urethra, rectum, throat), often produces no symptoms, and also contributes to PID and infertility when untreated. A growing concern with gonorrhea is antibiotic resistance, which has made some strains harder to treat over the years.
Syphilis progresses through distinct stages if untreated. It starts as a painless sore (called a chancre), then can cause rashes and flu-like symptoms in the second stage. After that, it may go dormant for years before eventually causing serious damage to the brain, heart, and other organs. Syphilis is especially dangerous during pregnancy, where it can cause stillbirth or severe birth defects. A single dose of penicillin cures early-stage syphilis.
Viral STIs: HPV, Herpes, and HIV
Viral STIs cannot be cured, but all of them can be managed, and one of them is largely preventable through vaccination.
HPV (human papillomavirus) is the most widespread STI overall. Most sexually active people will get at least one type of HPV in their lifetime. There are many strains: low-risk types cause genital or oral warts, while high-risk types can lead to cervical, anal, penile, and throat cancers. A person can carry HPV for years without any symptoms. When warts do develop, they can be removed through freezing, topical treatments, or minor procedures. Precancerous cell changes caught on screening can be removed before they progress.
The HPV vaccine is the most effective prevention tool available. The CDC recommends routine vaccination at age 11 or 12, though it can start as early as 9. Catch-up vaccination is recommended for anyone through age 26 who wasn’t vaccinated earlier. Adults between 27 and 45 may also benefit, depending on their situation. Kids who start the series before age 15 need only two doses spaced 6 to 12 months apart, while those starting at 15 or older need three doses.
Genital herpes is caused by herpes simplex virus, usually type 2. It produces recurring outbreaks of painful blisters or sores around the genitals, though many people have mild symptoms they never recognize as herpes. Antiviral medication reduces the frequency and severity of outbreaks and lowers the chance of passing the virus to a partner. There is no vaccine for herpes.
HIV attacks the immune system’s infection-fighting cells, gradually weakening the body’s ability to fight off illness. Without treatment, HIV progresses to AIDS. With modern antiretroviral therapy, however, people with HIV can live long, healthy lives and reach an undetectable viral load, meaning they effectively cannot transmit the virus to sexual partners. Prevention has also advanced significantly: PrEP (pre-exposure prophylaxis) reduces the risk of getting HIV from sex by about 99 percent when taken as prescribed.
Parasitic STIs: Trichomoniasis and Pubic Lice
Trichomoniasis is the most common nonviral STI worldwide, affecting an estimated 2.6 million people in the United States. It’s caused by a single-celled parasite. In women, it can produce a yellow-green, foul-smelling vaginal discharge and vulvar irritation. In men, it sometimes causes urinary discomfort. But the defining feature of trichomoniasis is how often it hides: 70 to 85 percent of infected people have minimal or no symptoms. It’s curable with a short course of oral antiparasitic medication.
Pubic lice (sometimes called “crabs”) are tiny insects that attach to coarse body hair, primarily in the genital area. They cause intense itching and are spread through close physical contact. Over-the-counter treatments typically clear them up.
Why Many STIs Go Unnoticed
One of the most important things to understand about STIs is that the majority of them can be completely silent. A large meta-analysis of women in multiple countries found that roughly 61 percent of chlamydia infections, 53 percent of gonorrhea infections, and 57 percent of trichomoniasis infections produced no symptoms. Herpes and HPV are similarly quiet in many carriers. This is why STIs spread so efficiently: people pass them on without ever knowing they’re infected.
This also means you can’t rely on symptoms to tell you whether you or a partner has an STI. Regular screening is the only reliable way to catch infections early, especially since untreated bacterial STIs can cause permanent reproductive damage even when they produce no noticeable signs.
Testing Windows to Know About
If you’ve had a potential exposure, getting tested too early can produce a false negative because the infection hasn’t built up enough to detect. Each STI has a different testing window:
- Chlamydia and gonorrhea: Typically detectable within 1 to 2 weeks after exposure.
- Syphilis: A blood test catches most infections at 1 month, and nearly all by 3 months.
- HIV: A blood test using antigen/antibody methods catches most infections at 2 weeks, with nearly all detected by 6 weeks. Oral swab tests take longer, catching most at 1 month and nearly all by 3 months.
- Hepatitis B: Blood tests typically detect infection at 3 to 6 weeks.
If your first test comes back negative but your exposure was recent, retesting after the full window has passed gives you a more reliable result.
Bacterial vs. Viral: What Treatment Looks Like
The most practical distinction between STIs is whether they’re curable. Bacterial and parasitic infections (chlamydia, gonorrhea, syphilis, trichomoniasis) are all treated with antibiotics or antiparasitic drugs. Treatment is usually short, sometimes just a single dose, and clears the infection completely. You can, however, get reinfected.
Viral infections (HPV, herpes, HIV) stay in the body. That doesn’t mean they’re unmanageable. HPV often clears on its own within a couple of years, herpes outbreaks become less frequent over time with or without medication, and HIV treatment has turned what was once a fatal diagnosis into a chronic but controllable condition. The key with all viral STIs is monitoring: regular cervical screening for HPV, antiviral medication for herpes when needed, and consistent antiretroviral therapy for HIV.
Prevention Beyond Condoms
Condoms remain one of the most effective and accessible tools for reducing STI transmission, but they aren’t the only option. The HPV vaccine prevents infection from the strains responsible for most HPV-related cancers and genital warts. PrEP is a daily pill (or long-acting injection) that offers near-complete protection against HIV. Regular screening catches curable infections before they cause damage or spread further. And for people diagnosed with herpes or HIV, staying on medication dramatically lowers the risk of transmitting the virus to partners.