means A devastating epidemic disease — especially the bacterial infection that killed millions in medieval Europe — or, more loosely, any widespread affliction or relentless torment.

from From Latin plaga, meaning a 'blow,' 'wound,' or 'stroke' — the same sense behind being struck down. It came into English through Latin and Old French, carrying the idea that disaster lands like a hit. The biblical 'plagues' of Egypt sharpened the word's apocalyptic flavor, and the verb sense ('to plague someone') keeps the original bruising metaphor alive: to be plagued is to be repeatedly struck.