Real Dictionary

epidemic
If there is an epidemic of a particular disease somewhere, it affects a very large number of people there and spreads quickly to other areas.
countable noun: If an activity that you disapprove of is increasing or spreading rapidly, you can refer to this as an epidemic of that activity.

epidemic in American English
prevalent and spreading rapidly among many individuals in a community at the same time
adjective: affecting many persons at the same time, and spreading from person to person in a locality where the disease is not permanently prevalent
noun: a temporary prevalence of a disease

epidemic in British English
adjective: (esp of a disease) attacking or affecting many persons simultaneously in a community or area
noun: a widespread occurrence of a disease