Source : Webster's Revised Unabridged Dictionary (1913)

Contagion \Con*ta"gion\, n. [L. contagio: cf. F. contagion. See
   {Contact}.]
   1. (Med.) The transmission of a disease from one person to
      another, by direct or indirect contact.

   Note: The term has been applied by some to the action of
         miasmata arising from dead animal or vegetable matter,
         bogs, fens, etc., but in this sense it is now
         abandoned. --Dunglison.

               And will he steal out of his wholesome bed To
               dare the vile contagion of the night? --Shak.

   2. That which serves as a medium or agency to transmit
      disease; a virus produced by, or exhalation proceeding
      from, a diseased person, and capable of reproducing the
      disease.

   3. The act or means of communicating any influence to the
      mind or heart; as, the contagion of enthusiasm. ``The
      contagion of example.'' --Eikon Basilike.

            When lust . . . Lets in defilement to the inward
            parts, The soul grows clotted by contagion.
                                                  --Milton.

   4. Venom; poison. [Obs.] ``I'll touch my point with this
      contagion.'' --Shak.

   Syn: See {Infection}.

Source : WordNet®

contagion
     n 1: any disease easily transmitted by contact [syn: {contagious
          disease}]
     2: an incident in which an infectious disease is transmitted
        [syn: {infection}, {transmission}]
     3: the communication of an attitude or emotional state among a
        number of people; "a contagion of mirth"; "the infection
        of his enthusiasm for poetry" [syn: {infection}]