Short answer
An STI is a sexually transmitted infection. A pathogen is present in the body.
An STD is a sexually transmitted disease. The infection is causing symptoms or illness.
Every STD starts as an STI, but most STIs never become diseases: chlamydia usually sits silent, HPV usually clears unnoticed, herpes sheds without sores most days.
The distinction is real and useful. And in everyday reading, the terms overlap heavily unless the source defines them.
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See Tests & Prices →Why usage shifted toward STI
Because most sexually transmitted infections are frequently asymptomatic, “infection” describes reality more accurately, and carries less stigma, than “disease.” The CDC now organizes its resources around “STI,” while MedlinePlus still titles its hub “Sexually Transmitted Diseases.” Both are legitimate; neither is wrong.
What matters is recognizing when a source uses one term for precision (screening guidance favors STI; surveillance reports often say STD) versus interchangeably (most journalism, most conversation).
When each term fits best
Use STI when discussing transmission (“HPV transmits through skin contact”), screening (“yearly chlamydia STI screening”), silent infection, and prevention.
Use STD when discussing symptomatic illness (“PID is a complication of untreated STIs”. Note even clinicians mix them mid-sentence there), historical data labeled as STD statistics, and MedlinePlus-style disease framing.
In practice: default to STI in your own speech (less stigma, more accuracy), understand both in others’.
Concrete examples that lock the distinction
Chlamydia detected on a routine screen with zero symptoms: STI, not STD.
The same chlamydia ascending into pelvic inflammatory disease with fever and pain: now STD territory.
HPV present without warts or abnormal Pap: STI. Genital warts requiring treatment: disease manifesting.
HIV controlled on treatment without illness: infection managed. The terminology matters less than the monitoring.
Why it matters for testing behavior
The terminology shift encodes a medical insight: absence of symptoms never equals absence of infection.
People who wait for “disease” to appear miss the “infection” window when screening works best and transmission runs highest.
Screening exists precisely because STIs spread silently. The term is the reminder.
See What Does Asymptomatic Mean for STIs?.
When to seek care
Terminology questions never delay testing. But while here: routine screening on schedule regardless of symptoms, prompt evaluation for any discharge/sore/rash/pain, and clinician conversations using whichever term comes naturally (they’ll understand both).