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What is definition of epidemic? of outbreak? of cluster? What do these terms mean to an epidemiologist? What do they mean to the public? To a reporter?
Real versus artifact – not all apparent increases in disease reflect true increases. Some are due to artifacts of reporting – change in diagnostic test or case definition, improved reporting, heightened awareness, etc.
Compared to what – epidemic is occurrence of more cases of disease than you’d expect in a given area or among a specific group of people over a particular period of time. How do you figure out what’s the expected level of disease? For most communicable diseases, use local surveillance data. For others, may need hospital discharge data, mortality data, cancer registry data, etc.