Pronunciation: (in"flOO-en'zu),[key] —n. 1. Pathol.an acute, commonly epidemic disease, occurring in several forms, caused by numerous rapidly mutating viral strains and characterized by respiratory symptoms and general prostration. Cf. flu. 2. Vet. Pathol.an acute, contagious disease occurring in horses and swine, characterized by fever, depression, and catarrhal inflammations of the eyes, nasal passages, and bronchi, and caused by a virus.