- a specific variety of influenza, usually named for its point of dissemination or its animal vector: Hong Kong flu; swine flu.
- the flu short for influenza
- any of various viral infections, esp a respiratory or intestinal infection
1839, flue, shortening of influenza. Spelling flu attested from 1893. For choice of middle syllable, not a common method of shortening in English, Weekley compares tec for detective, scrip for subscription.
- See influenza.