ague: [14] In its origins, ague is the same word as acute. It comes from the Latin phrase febris acuta ‘sharp fever’ (which found its way into Middle English as fever agu). In the Middle Ages the Latin adjective acuta came to be used on its own as a noun meaning ‘fever’; this became aguē in medieval French, from which it was borrowed into English. From the end of the 14th century ague was used for ‘malaria’ (the word malaria itself did not enter the language until the mid 18th century). => acute
ague (n.)
"malarial fever," c. 1300, from Old French ague "acute fever," from Medieval Latin (febris) acuta "sharp (fever)," with fem. of acutus "sharp" (see acute).