The Collaborative International Dictionary of English v.0.48:

Contagion \Con*ta"gion\ (k[o^]n*t[=a]"j[u^]n), 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.
      [1913 Webster]

   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.
         [1913 Webster]

               And will he steal out of his wholesome bed
               To dare the vile contagion of the night? --Shak.
         [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

            When lust . . .
            Lets in defilement to the inward parts,
            The soul grows clotted by contagion.  --Milton.
      [1913 Webster]

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

   Syn: See Infection.
        [1913 Webster]

WordNet (r) 3.0 (2006):

contagion
    n 1: any disease easily transmitted by contact [syn: contagious
         disease, contagion]
    2: an incident in which an infectious disease is transmitted
       [syn: infection, contagion, 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: contagion, infection]