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From 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]