Epidemic Defined.

adj.
1. Spreading rapidly and extensively by infection and affecting many individuals in an area or a population at the same time: an epidemic outbreak of influenza.
2. Widely prevalent: epidemic discontent.
n.
1. An outbreak of a contagious disease that spreads rapidly and widely.
2. A rapid spread, growth, or development: an unemployment epidemic.

Source: Answers.com

********************************************

Epidemic \Ep`i*dem"ic\, Epidemical \Ep`i*dem"ic*al\, a. [L. epidemus, Gr. ?, ?, among the people, epidemic; ? in + ? people: cf. F. ['e]pid['e]mique. Cf. Demagogue.]

1. (Med.) Common to, or affecting at the same time, a large number in a community; -- applied to a disease which, spreading widely, attacks many persons at the same time; as, an epidemic disease; an epidemic catarrh, fever, etc. See Endemic.

2. Spreading widely, or generally prevailing; affecting great numbers, as an epidemic does; as, epidemic rage; an epidemic evil.

It was the epidemical sin of the nation. --Bp. Burnet.

Source: Webster's Revised Unabridged Dictionary (1913)