'plague-causing' is a Modern English compound formed from 'plague' + present participle 'causing' (from 'cause'). 'plague' ultimately comes via Old French from Latin 'plaga' meaning 'stroke, blow' and later 'disease'; 'cause' comes from Latin 'causa' meaning 'reason' or 'cause'.

'plague' came from Latin 'plaga' → Old French 'plague' → Middle English 'plage' → Modern English 'plague'. 'cause' came from Latin 'causa' → Old French 'cause' → Middle English 'cause'. The compound 'plague-causing' is a descriptive formation in Modern English.

Initially 'plaga' meant 'a blow or strike' and later referred to a contagious disease; 'causa' meant 'reason' or 'source'. The combined expression came to mean 'bringing or producing plague' and is also used figuratively to mean 'bringing widespread disaster'.