When is an epidemic an epidemic?

Background: The large number of cases of West Nile fever diagnosed in Israel in 2000 once again brought into focus the confusion that frequently accompanies the use of the term "epidemic". Objectives: To examine the different definitions of the term "epidemic" and to propose ways in which it can be used to both improve communication among professionals and provide the public with a better sense of the associated risks. Methods: The literature was reviewed for the various definitions of the terms "epidemic" and "outbreak". Sources included popular and medical dictionaries, ancient documents, epidemiology texts, legal texts, and the medical literature. Results: The term epidemic is variously defined. The broad definition given by epidemiologists - namely, more disease than is anticipated by previous experience - is less meaningful to the general public. In some ways it conflicts with the definitions found in the popular literature, which generally imply danger to the public and a very large number of victims. Conclusions: The interpretation of the term epidemic may vary according to the context in which it is used. For risk communication, we suggest that every effort be made to add descriptive terms that characterize the epidemic.