The CDC's official definition of an epidemic is: "The occurrence of more cases of disease than expected in a given area or among a specific group of people over a particular period of time."
Not very definitive. But "Webster" isn't much better:
occurring suddenly in numbers clearly in excess of normal expectancy, in contrast to endemic or sporadic. The term is used especially of infectious diseases but is also applied to any disease, injury, or other health-related event occurring in such outbreaks.
This may shed more light on the subject for you. It applies specifically to HIV/AIDS but the principles apply across the board (and many in the fields of medicine and Public Health feel tht JHU's Blomberg School of Public Health is the world standard).