What Is an STI?

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

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.

Bacterial STIs: curable in theory, complicated by resistance

Bacterial STIs — chlamydia, gonorrhea, syphilis — are generally curable with antibiotics, but cure depends on the right drug. According to CDC, uncomplicated gonorrhea is now treated with ceftriaxone 500 mg intramuscularly as a single dose for adults under 150 kg. If chlamydia has not been ruled out, the recommended companion regimen is doxycycline 100 mg by mouth twice daily for 7 days; azithromycin is no longer first-line because of resistance.

Chlamydia typically responds to doxycycline 100 mg twice daily for 7 days. Syphilis remains penicillin-sensitive: the stage determines the dose and duration of benzathine penicillin G. No broad-spectrum shortcut is reliable.

Antimicrobial resistance makes even these curable infections a public health problem, so treatment should follow current CDC regimens rather than older antibiotic combinations. Retesting after treatment for gonorrhea and chlamydia is often advised at 3 months, not because the drugs failed but because reinfection is common.

Throat and rectal infections: why site matters

STIs are not limited to the genitals. Gonorrhea and chlamydia can infect the throat and rectum through oral or anal sex, and these extragenital infections are usually asymptomatic, per CDC. A urine test or vaginal swab alone can miss them; throat and rectal swabs are needed if those sites were exposed.

Ulcerative and viral STIs also have specific site patterns. Herpes simplex virus and syphilis can enter through breaks in skin or mucosa anywhere they contact an infectious sore or rash. Human papillomavirus transmits through intimate skin-to-skin contact, so condoms reduce but do not fully eliminate HPV exposure.

Knowing the site matters because symptoms — a sore throat, rectal discomfort, or a painless ulcer — may not look like a classic sexually transmitted infection, and because undetected extragenital infection can still be transmitted to partners.

Window periods: when a negative result is reliable

A negative STI test does not rule out a very recent exposure. Each pathogen has a window period between exposure and when a test can reliably detect it. According to CDC, chlamydia and gonorrhea NAAT tests are usually reliable by 1 to 2 weeks after exposure. Syphilis blood tests often become reactive within 1 to 3 months; a painless chancre may appear around 3 weeks but is sometimes absent or unnoticed.

For HIV, laboratory antigen/antibody tests detect most infections by 18 to 45 days after exposure, while some antibody tests can take up to 90 days, per CDC. If a person tests before the window period closes, a negative result should be repeated later. Timing prevents false reassurance and helps plan accurate retesting.