1. Having qualities that may infect; communicable or caused by infection; pestilential; epidemic; as, an infectious fever; infectious clothing; infectious water; infectious vices.
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.
3. (Law) Contaminating with illegality; exposing to seizure and forfeiture.
Contraband articles are said to be of an infectious nature.
4. Capable of being easily diffused or spread; sympathetic; readily communicated; as, infectious mirth.
The laughter was so genuine as to be infectious.
Adj. 1. infectious - caused by infection or capable of causing infection; "viruses and other infective agents"; "a carrier remains infective without himself showing signs of the disease"
3. infectious - of or relating to infection; "infectious hospital"; "infectious disease"