Source : Webster's Revised Unabridged Dictionary (1913)

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.