What is epidemic?
epidemic is A group of cases of a specific disease or illness clearly in excess of what one would normally expect in a particular geographic area. There is no absolute criterion for using the term epidemic; as standards and expectations change, so might the definition of an epidemic, e.g. an epidemic of violence.
source: A GLOSSARY OF TERMS FOR COMMUNITY HEALTH CARE AND SERVICES FOR OLDER PERSONS. World Health Organization, 2004.