A sexually transmitted infection (STI) is an infection caused by a virus, bacterium, parasite, or fungus that spreads through sexual contact. More than one million new STIs are acquired every day worldwide. Many people carry an STI without knowing it, which is why understanding how these infections work, how they spread, and how to prevent them matters for anyone who is sexually active.
STI vs. STD: Why the Name Changed
You’ve probably seen both “STI” and “STD” used interchangeably, but they mean slightly different things. An STI refers to the presence of a pathogen in your body, whether or not it’s causing problems. An STD means that infection has progressed enough to produce actual symptoms or disease. Most health organizations, including the CDC, now prefer “STI” because the goal is to catch and treat infections before they ever become diseases. The shift also helps reduce stigma: calling something an “infection” rather than a “disease” more accurately reflects what’s happening in most people’s bodies.
The Eight Most Common STIs
Eight pathogens account for the vast majority of sexually transmitted infections globally. They fall into three categories based on what causes them.
Bacterial STIs include chlamydia, gonorrhea, and syphilis. These are caused by bacteria and can typically be cured with antibiotics, though gonorrhea is becoming increasingly resistant to treatment. The World Health Organization has warned that we may eventually run out of effective drugs for multi-drug resistant gonorrhea.
Viral STIs include HIV, herpes (HSV), human papillomavirus (HPV), and hepatitis B. Viral infections cannot be cured with antibiotics. Some, like herpes and HIV, stay in the body permanently but can be managed with medication to reduce symptoms and prevent transmission. HPV and hepatitis B are both preventable with vaccines, which is significant because both can lead to cancer if left unchecked.
Parasitic STIs include trichomoniasis, which is caused by a tiny parasite and is fully curable with medication. It’s actually one of the most common STIs in the world, though many people have never heard of it.
How STIs Spread
Most STIs spread through vaginal, anal, or oral sex. But penetrative sex isn’t the only route. Herpes and HPV transmit through skin-to-skin contact, meaning condoms reduce risk but don’t eliminate it entirely since they don’t cover all affected skin. Syphilis spreads through direct contact with a sore, which can appear on the genitals, mouth, or rectum.
Several STIs can also pass from a pregnant person to their baby during pregnancy or childbirth. Chlamydia, gonorrhea, syphilis, HIV, herpes, HPV, and hepatitis B all carry this risk, which is why STI screening is a routine part of prenatal care.
Research has also identified pathogens not traditionally classified as STIs that can spread through intimate contact, including certain skin fungi and enteric bacteria like Shigella. The line between “sexually transmitted” and “transmitted during sex” is blurrier than most people realize.
Most People With an STI Have No Symptoms
This is the single most important thing to understand about STIs: the majority of infections produce no symptoms at all. Research on chlamydia found that roughly 77% of all cases never caused noticeable symptoms. For gonorrhea, that number was about 45%. The overwhelming reason these infections go untreated isn’t that people ignore symptoms; it’s that they never have symptoms to ignore.
When symptoms do appear, they vary widely depending on the infection. Common signs include unusual discharge from the genitals, burning during urination, sores or blisters on or around the genitals or mouth, itching, and pain during sex. Some infections like syphilis progress through distinct stages, with early sores that heal on their own before the infection quietly advances to more serious phases. The absence of symptoms at any point does not mean the infection has cleared.
What Happens If an STI Goes Untreated
Untreated STIs can cause serious long-term health problems, particularly for women. Chlamydia and gonorrhea can lead to pelvic inflammatory disease (PID), a condition where the infection spreads to the uterus and fallopian tubes. PID can cause chronic pelvic pain, ectopic pregnancy (where a fertilized egg implants outside the uterus), and infertility. Because PID is often mild or asymptomatic itself, damage to the reproductive tract can accumulate silently over time. Early treatment of symptomatic PID lowers the risk of infertility, but many cases are never caught.
Herpes increases vulnerability to HIV. A meta-analysis found that in populations with herpes (HSV-2), up to 50% of HIV infections could be attributed to the presence of herpes. Genital ulcers caused by herpes create entry points that make HIV transmission more likely in both directions.
HPV, if it persists, can lead to cervical cancer and several other cancers. Syphilis left untreated for years can damage the brain, nerves, eyes, and heart. Hepatitis B can cause chronic liver disease and liver cancer. These outcomes are largely preventable with early detection and treatment or vaccination.
How STI Testing Works
There’s no single test that checks for every STI at once. Testing involves different sample types depending on which infection your provider is looking for.
- Blood tests are used for HIV, syphilis, hepatitis B, and sometimes herpes.
- Urine tests can detect chlamydia, trichomoniasis, and sometimes gonorrhea.
- Swab tests collect samples from the site of potential infection (genitals, cervix, urethra, or throat) and are used for HPV, chlamydia, gonorrhea, and herpes.
If you ask for “an STI test,” it’s worth confirming exactly which infections are being checked. Standard panels vary between clinics and providers, and some infections like herpes and HPV are not always included in routine screening unless you have symptoms or specific risk factors. Being direct about what you want tested gives you the clearest picture of your status.
Prevention: What Actually Works
Condoms and dental dams significantly reduce the risk of most STIs when used consistently and correctly. They’re most effective against infections spread through bodily fluids (like chlamydia, gonorrhea, and HIV) and somewhat less effective against those spread through skin contact (like herpes and HPV), since the virus can live on skin not covered by a condom.
Vaccination is the most effective prevention tool for the STIs that have vaccines. HPV vaccination is recommended for everyone starting at age 11 or 12 and is available through age 26 for anyone not previously vaccinated. Adults aged 27 to 45 can also get the vaccine after discussing it with their provider. Hepatitis B vaccination is recommended for all unvaccinated, sexually active people with more than one partner, as well as anyone being evaluated for an STI. Hepatitis A vaccination is also recommended for certain groups.
Routine screening remains the backbone of STI control, precisely because so many infections are silent. Regular testing, open communication with sexual partners, and reducing the number of concurrent partners all lower your risk profile meaningfully.