What is an Infectious Disease?

Infectious, or communicable, diseases are illnesses caused by a specific agent that is transmitted from an infected person, animal or reservoir to a susceptible host.

Numerous terms are used to describe the occurrence of infectious diseases in a defined population at a particular point in time.

Source: Kasia Bialasiewicz / Bigstock

• Prevalence - portion of the people with the disease in a particular population
• Incidence - describes the number of new cases in a population within a certain time interval
• Epidemic - sometimes referred to as an outbreak, an excess of cases above a normal amount for a specific population and region
• Endemic - refers to the constant presence of a disease in a region
• Emerging - diseases may be caused by a variety of pathogens considered undergoing change