1. 
[syn: infectious, infective]
2.  easily spread; 
- Example: "fear is exceedingly infectious children catch it from their elders"- Bertrand Russell; 
3.  of or relating to infection; 
- Example: "infectious hospital"
- Example: "infectious disease"
The Collaborative International Dictionary of English v.0.48:
Infectious \In*fec"tious\, a. [Cf. F. infectieux.]
   [1913 Webster]
   1. Having qualities that may infect; communicable or caused
      by infection; pestilential; epidemic; as, an infectious
      fever; infectious clothing; infectious water; infectious
      vices.
      [1913 Webster]
            Where the infectious pestilence.      --Shak.
      [1913 Webster]
   2. Corrupting, or tending to corrupt or contaminate;
      vitiating; demoralizing.
      [1913 Webster]
            It [the court] is necessary for the polishing of
            manners . . . but it is infectious even to the best
            morals to live always in it.          --Dryden.
      [1913 Webster]
   3. (Law) Contaminating with illegality; exposing to seizure
      and forfeiture.
      [1913 Webster]
            Contraband articles are said to be of an infectious
            nature.                               --Kent.
      [1913 Webster]
   4. Capable of being easily diffused or spread; sympathetic;
      readily communicated; as, infectious mirth.
      [1913 Webster]
            The laughter was so genuine as to be infectious.
                                                  --W. Black.
   Syn: See Contagious.
        [1913 Webster]
WordNet (r) 3.0 (2006):
infectious
    adj 1: caused by infection or capable of causing infection;
           "viruses and other infective agents"; "a carrier remains
           infective without himself showing signs of the disease"
           [syn: infectious, infective]
    2: easily spread; "fear is exceedingly infectious; children
       catch it from their elders"- Bertrand Russell [ant:
       noninfectious]
    3: of or relating to infection; "infectious hospital";
       "infectious disease"
Moby Thesaurus II by Grady Ward, 1.0:
40 Moby Thesaurus words for "infectious":
   catching, communicable, contagious, deadly, destructive, endemic,
   envenomed, epidemial, epidemic, epiphytotic, epizootic, infective,
   inoculable, irresistible, malign, malignant, mephitic, miasmal,
   miasmatic, miasmic, noxious, pandemic, pestiferous, pestilent,
   pestilential, poisonous, sporadic, spreading, sympathetic, taking,
   toxic, toxicant, toxiferous, transmissible, venenate, veneniferous,
   venenous, venomous, virulent, zymotic