Case definition with an example:

A case definition includes criteria for person, place, time, and clinical features. These should be specific to the outbreak under investigation. 

"Person" describes key characteristics the patients share in common. For example, this description may include: age, sex, race, occupation and exclusion criteria (e.g., “persons with no history of X disease”). 

"Place" typically describes a specific geographic location (state, county) or facility associated with the outbreak (X nursing home, Y high school).

 "Time" is used to delineate a period of time associated with illness onset for the cases under investigation. Limiting the time period enables exclusion of similar illnesses which are unrelated to the outbreak of interest. 

Initially, "clinical features" should be simple and objective (e.g., sudden onset of fever and cough). The clinical criteria may later be characterized by the presence of specific laboratory findings.