What Are Sexually Transmitted Infections?
Sexually transmitted infections (STIs) — sometimes still referred to as sexually transmitted diseases (STDs) — are infections passed primarily through sexual contact. The term "infection" is now preferred by most health organizations because many people carry and can transmit these pathogens without ever developing noticeable disease.
STIs are caused by three broad categories of microorganisms:
- Bacteria — responsible for chlamydia, gonorrhea, and syphilis
- Viruses — including HIV, herpes simplex virus (HSV), human papillomavirus (HPV), and hepatitis B
- Parasites — such as Trichomonas vaginalis, the organism behind trichomoniasis
Understanding which type of organism causes a particular STI matters because it directly shapes how the infection is treated. See the Infectious Disease Glossary for plain-language definitions of terms used throughout this guide.
Window period
The time between when a person is first infected and when a test can reliably detect the infection. Testing too early within this period can produce a false-negative result.
Asymptomatic
Having an infection without experiencing any noticeable signs or symptoms. Many STIs are asymptomatic, making routine testing essential.
Pathogen
Any microorganism — such as a bacterium, virus, or parasite — that can cause disease in humans.
Transmission
The process by which an infectious agent passes from one person to another, whether through bodily fluids, skin contact, or other routes.
PrEP
Pre-exposure prophylaxis — a prescribed medication regimen that significantly reduces the risk of contracting HIV when taken consistently by people at higher risk.
Perinatal transmission
When an infection passes from a parent to a child during pregnancy, labor, delivery, or breastfeeding.
This article provides general health information and is not a substitute for advice from a qualified healthcare professional.
How STIs Spread
Most STIs spread through contact with infected bodily fluids — including semen, vaginal secretions, blood, or breast milk — or through direct skin-to-skin contact with an infected area. The specific route depends on the pathogen involved.
- Vaginal, anal, or oral sex is the most common transmission route for the majority of STIs.
- Skin-to-skin contact without penetration can transmit herpes and HPV.
- Blood-to-blood contact — through shared needles or, less commonly today, blood transfusions — can transmit HIV and hepatitis B and C.
- Perinatal transmission (from parent to child during pregnancy, childbirth, or breastfeeding) is possible with HIV, syphilis, and herpes, among others.
Importantly, STIs are not spread through casual contact such as hugging, sharing food, using the same toilet, or being in the same room as an infected person. For a broader look at how infectious agents move between people, see our guide on how your immune system responds to infection.
Common STIs and Their Symptoms
One of the most important facts about STIs is that many produce no symptoms at all, or symptoms so mild they go unnoticed. The table below summarizes the most prevalent STIs and their typical presentations.
| STI | Cause | Common Symptoms (if any) |
|---|---|---|
| Chlamydia | Bacteria | Often none; discharge, burning urination, pelvic pain |
| Gonorrhea | Bacteria | Often none; discharge, painful urination, sore throat (oral) |
| Syphilis | Bacteria | Painless sore (chancre), rash, flu-like symptoms in later stages |
| Herpes (HSV) | Virus | Often none; blisters or sores on genitals or mouth |
| HPV | Virus | Usually none; some strains cause genital warts |
| HIV | Virus | Flu-like illness shortly after infection; often asymptomatic for years |
| Trichomoniasis | Parasite | Itching, burning, discharge with odor; often none |
Do Not Self-Diagnose Based on Symptoms Alone
Many STI symptoms — such as discharge, sores, or burning — overlap with non-STI conditions, and many STIs cause no symptoms at all. Only laboratory testing can confirm or rule out an infection. Self-diagnosis or self-treating without professional guidance can delay appropriate care and increase the risk of complications.
If you notice any unusual symptoms in the genital area or are concerned about a potential exposure, contact a healthcare provider rather than waiting to see if symptoms resolve on their own.
Testing: What to Expect
Because symptoms are so often absent, routine testing is the only reliable way to know your status. The Centers for Disease Control and Prevention (CDC) recommends that all sexually active adults discuss their testing needs with a healthcare provider, with certain groups advised to test annually or more frequently.
Testing methods vary by infection and may include:
- Urine samples — commonly used for chlamydia and gonorrhea
- Blood tests — used for HIV, syphilis, herpes antibodies, and hepatitis B and C
- Swabs — taken from the throat, rectum, or genitals depending on sexual practices
- Physical examination — a clinician may visually assess sores or warts
Ask About Window Periods Before You Test
Different STIs become detectable at different times after exposure. Tell your healthcare provider when potential exposure occurred so they can recommend the most accurate testing window for each infection. Retesting after the window period has passed may be necessary for a conclusive result.
Window periods — the time between infection and when a test becomes accurate — vary by STI. Your provider can advise on the best timing for each test based on when potential exposure occurred.
Testing is available through primary care offices, sexual health clinics, and community health centers. Confidential and anonymous options exist in many areas. For general practices that support overall infection resilience, see everyday habits that support infection prevention.
Prevention Strategies That Work
No single prevention method is 100% effective, but combining strategies substantially lowers risk.
- Barrier methods — External (male) condoms, when used consistently and correctly, significantly reduce transmission of most STIs. Internal (female) condoms are also effective. Dental dams reduce risk during oral-vaginal or oral-anal contact.
- Vaccination — Vaccines for HPV and hepatitis B are safe, effective, and widely recommended. HPV vaccination is most effective when given before potential exposure, typically in adolescence, but can still offer benefit in adulthood.
- PrEP — Pre-exposure prophylaxis is a daily medication that, when taken consistently, is highly effective at preventing HIV transmission in people at elevated risk. A healthcare provider must assess eligibility and prescribe it.
- Regular testing and open communication — Knowing your own status and discussing it openly with partners is itself a prevention strategy.
- Mutual monogamy — A long-term relationship in which both partners have tested negative and remain exclusive reduces risk, though testing is still advisable before discontinuing barrier methods.
For those exploring reproductive health more broadly, the Reproductive Wellness hub covers related topics including contraception and sexual health for women.
This article is for informational purposes only and is not medical advice. Always consult a qualified healthcare professional for guidance on testing, diagnosis, and treatment.
