1. [n]  a widespread outbreak of an infectious disease; many people are infected at the same time
2. [adj]  (especially of medicine) of disease or anything resembling a disease; attacking or affecting many individuals in a community or a population simultaneously; "an epidemic outbreak of influenza"

Epidemic
Discusses how epidemics get started and spread, what can be done to prevent them, and some of history's greatest, including the Black Death and AIDS

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1. \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.
   

2. \Ep`i*dem"ic\, n. [Cf. {Epidemy}.]
   1. (Med.) An epidemic disease.
   2. Anything which takes possession of the minds of people as
      an epidemic does of their bodies; as, an epidemic of
      terror.