1. (a.) Having qualities that may infect; communicable or caused by infection; pestilential; epidemic; as, an infectious fever; infectious clothing; infectious air; infectious vices.

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

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

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