\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}.