Communicable or infectious diseases, the third leading cause of death in the U.S., are generally caused by viruses, bacteria or parasites and transmitted by direct or indirect contact through people, animals, insects, food, air, water, and even contaminated surface areas.
Each disease can be distinguished by its method of transmission, ease of contagiousness, how long the pathogen can stay alive and how dangerous the resulting disease is to the person or animal contracting it.
Outbreak - more disease cases than what would normally occur in a specific location or season.
Epidemic - increased cases of a disease that are spreading widely and rapidly.
Pandemic - an epidemic of infectious diease spreading between humans worldwide.