Infectious diseases (also known as transmissible diseases or communicable diseases) are clinically evident illnesses caused by microorganisms, such as bacteria, viruses, fungi, or parasites that can be spread - directly or indirectly - from one vector to another. Some infectious diseases are caused by person-to-person contact, others are passed via animal or insect bite, and still others are caused by ingestion of infected, contaminated food, water, or other environmental exposures.