Source : Webster's Revised Unabridged Dictionary (1913)

Infectious \In*fec"tious\, a. [Cf. F. infectieux.]
   1. Having qualities that may infect; communicable or caused
      by infection; pestilential; epidemic; as, an infectious
      fever; infectious clothing; infectious air; infectious
      vices.

            Where the infectious pestilence.      --Shak.

   2. Corrupting, or tending to corrupt or contaminate;
      vitiating; demoralizing.

            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.

   3. (Law) Contaminating with illegality; exposing to seizure
      and forfeiture.

            Contraband articles are said to be of an infectious
            nature.                               --Kent.

   4. Capable of being easily diffused or spread; sympathetic;
      readily communicated; as, infectious mirth.

            The laughter was so genuine as to be infectious.
                                                  --W. Black.

   Syn: See {Contagious}.

Parotitis \Par`o*ti"tis\, n. [NL. See {Parotid}, and {-itis}.]
   (Med.)
   Inflammation of the parotid glands.

   {Epidemic}, or {Infectious}, {parotitis}, mumps.

Source : WordNet®

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: {infective}]
     2: easily spread; "fear is exceedlingly infectious; children
        catch it from their elders"- Bertrand Russell [ant: {noninfectious}]
     3: of or relating to infection; "infectious hospital";
        "infectious disease"