A closely grouped series of events or cases of a disease or other health-related phenomena with well-defined distribution patterns in relation to time or place (or both) is termed:

This would be termed as an epidemic. For example; the Spanish Flu was known as an epidemic. The Black Plague and other instances like this are also know (to me) as epidemics that either were in relation to time, place, or ethnicity and the phenomena's specificity as of such ...