Influenza is a contagious respiratory illness caused by a virus. It is commonly referred to as the FLU and is characterized by fever, sore throat, muscle aches, and fatigue. The three crossword solutions for the term influenza are AGUE (4 letters), GRIPPE (6 letters), and FLU (3 letters). AGUE is an old-fashioned term for a fever or chill, GRIPPE is a French word for influenza, and FLU is a shortened version of the word influenza.