Infectious

a.
  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.
  3. (Law) Contaminating with illegality; exposing to seizure and forfeiture.
  4. Capable of being easily diffused or spread; sympathetic; readily communicated; as, infectious mirth.