prodrome
A precursor or harbinger; also a warning event.
rare
These may prove the Prodromes […] to the ruin of our Monarchy.
An introductory or preliminary book or treatise.
An early sign or symptom (or set of signs and symptoms) warning of the onset of a disease.
Where it comes from
Probably by analogy with syndrome (pro- + -drome), but consistent with Ancient Greek προδρομή (prodromḗ, “running forward, sally, skirmish”).
Adapted from the Wiktionary entry prodrome (CC BY-SA 4.0).