What Is an STI?
An STI (sexually transmitted infection) is an infection caused by a pathogen passed mainly through sexual contact — vaginal, anal or oral sex, and intimate skin contact. STI is the umbrella term doctors now prefer over STD: it describes having the infection even before any disease symptoms appear. Common STIs include chlamydia, gonorrhea, syphilis, herpes, HPV, HIV and trichomoniasis.
What "sexually transmitted infection" actually means
Break the term down and it explains itself: sexually transmitted = passed between partners through sexual activity; infection = a germ — bacterium, virus, parasite or protozoan — that has entered your body and begun living there.
The WHO recognizes more than 30 sexually transmissible pathogens. Eight currently drive most of the global burden: four are curable (syphilis, gonorrhea, chlamydia, trichomoniasis) and four are viral and lifelong (HIV, herpes simplex virus, HPV, hepatitis B).
Why "infection" beats "disease" in modern usage
Infection comes first; disease may follow. Someone carrying HPV has an STI from day one, but only develops a disease if warts or precancerous changes appear. Most infected people never cross that line — which is exactly why clinics say STI. It's accurate, less stigmatizing, and reflects reality: a positive test doesn't automatically mean you're sick.
If you're untangling the vocabulary further, our dedicated comparison walks through STI vs STD in detail.
The STIs you're statistically most likely to meet
| STI | Type | Curable? | US scale |
|---|---|---|---|
| HPV | Virus | No (clears/vaccine) | ~43M current infections |
| Chlamydia | Bacterium | Yes | 1.6M reported cases (2023) |
| Gonorrhea | Bacterium | Yes | ~601K reported (2023) |
| Genital herpes | Virus | Treatable | Majority of adults carry HSV-1 |
| Trichomoniasis | Parasite | Yes | ~2.6M infections |
| Syphilis | Bacterium | Yes | ~209K all stages (2023) |
| HIV | Virus | Treatable | ~1.2M living with HIV |
| Hepatitis B | Virus | Vaccine prevents | ~580K–1.2M US chronic carriers est. |
The full ranked rundown with symptoms lives in our most common STDs guide.
How STI testing works
Testing is faster and easier than most people expect. A urine sample catches chlamydia and gonorrhea; one blood draw covers HIV, syphilis and hepatitis; swabs check throat, rectum or any visible sores. No pelvic exam required in most modern protocols — self-collected swabs are validated and common.
Results typically arrive within one to three business days. Start with our plain-language guides to how each test is performed and when each test turns positive after exposure.
Frequently asked questions
Sources & further reading
- 📚 WHO — Sexually transmitted infections fact sheet — www.who.int/news-room/fact-sheets/detail/sexually-transmitted-infections-(stis)
- 📚 CDC — About STIs — www.cdc.gov/std
- 📚 CDC — STI Surveillance 2023 — www.cdc.gov/std/statistics