The Collaborative International Dictionary of English v.0.48:

Contagious \Con*ta"gious\, a. [L. contagiosus: cf. F.
   contagieux.]
   1. (Med.) Communicable by contact, by a virus, or by a bodily
      exhalation; catching; as, a contagious disease.
      [1913 Webster]

   2. Conveying or generating disease; pestilential; poisonous;
      as, contagious air.
      [1913 Webster]

   3. Spreading or communicable from one to another; exciting
      similar emotions or conduct in others.
      [1913 Webster]

            His genius rendered his courage more contagious.
                                                  --Wirt.
      [1913 Webster]

            The spirit of imitation is contagious. --Ames.

   Syn: Contagious, Infectious.

   Usage: Although often used as synonyms, originally these
          words were used in very diverse senses; but, in
          general, a contagious disease has been considered as
          one which is caught from another by some near contact,
          by the breath, by bodily effluvia, etc.; while an
          infectious disease supposed some entirely different
          cause acting by a hidden influence, like the miasma of
          prison ships, of marshes, etc., infecting the system
          with disease. In either case, a pathogenic
          microorganism is the direct cause of the disease. This
          distinction, though not universally admitted by
          medical men, as to the literal meaning of the words,
          certainly applies to them in their figurative use.
          Thus we speak of the contagious influence of evil
          associates; their contagion of bad example, the
          contagion of fear, etc., when we refer to transmission
          by proximity or contact. On the other hand, we speak
          of infection by bad principles, etc., when we consider
          anything as diffused by some hidden influence.
          [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

contagious
    adj 1: easily diffused or spread as from one person to another;
           "a contagious grin"
    2: (of disease) capable of being transmitted by infection [syn:
       catching, communicable, contagious, contractable,
       transmissible, transmittable]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "contagious":
   assignable, catching, communicable, conductive, consignable,
   conveyable, deadly, destructive, endemic, envenomed, epidemial,
   epidemic, epiphytotic, epizootic, expressable, impartable,
   infectious, infective, inoculable, interchangeable, mailable,
   malign, malignant, mephitic, metathetic, miasmal, miasmatic,
   miasmic, movable, noxious, pandemic, pestiferous, pestilential,
   poisonous, portable, portative, removable, sporadic, spreading,
   taking, toxic, toxicant, toxiferous, transferable, transfusable,
   transmissible, transmissive, transmittable, transportable,
   transportative, transposable, venenate, veneniferous, venenous,
   venomous, virulent, zymotic