In epidemiology, an epidemic (επι (epi)- meaning "upon or above" and δεμος (demos)- meaning "people"), occurs when new cases of a certain disease, in a given human population, and during a given period, substantially exceed what is expected based on recent experience.:354 Epidemiologists often consider the term outbreak to be synonymous to epidemic, but the general public typically perceives outbreaks to be more local and less serious than epidemics:55, 354
An epidemic may be restricted to one locale, however if it spreads to other countries or continents and affects a substantial number of people, it may be termed a pandemic.:55 The declaration of an epidemic usually requires a good understanding of a baseline rate of incidence; epidemics for certain diseases, such as influenza, are defined as reaching some defined increase in incidence above this baseline. A few cases of a very rare disease may be classified as an epidemic, while many cases of a common disease (such as the common cold) would not.
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