Prodrome

Etymology

From Greek "prodromos" (a running before, a forerunner), combining "pro" (before) with "dromos" (a running, a course). The word entered medical usage to mark the heralding stage of a disease.

noun · /PROH-drohm/
An early symptom or set of symptoms that signals the onset of a disease before its full features appear.
What it names
The warning phase of an illness — the faint first signs — giving clinicians a term for the window before a condition fully declares itself.
Example
The tingling on his lip was the familiar prodrome of a cold sore, appearing a day before the blister.
Related idea
Distinct from a symptom proper, which marks the established illness; a prodrome is a symptom of what is coming. It differs, too, from a prognosis, which is a forecast of how the disease will unfold, not an early sign of its arrival.