This is almost the same as the dictionary definition for "epidemic," and it doesn't explain much when it comes to influenza -- a.k.a. the flu.

An epidemic of influenza is different from the dreaded pandemic that scientists and world health officials fear is nigh. We might see an epidemic of seasonal influenza during any given year. In fact, we just had one.

Flu reached epidemic levels in the U.S. for 10 weeks in a row during the 2004-2005 season. Records kept by the CDC show that during the week ending March 5, 2005, 8.9% of all deaths reported in 122 U.S. cities were due to influenza and pneumonia (a common complication of the flu).

The CDC's definition of a flu epidemic relates to the percentage of deaths in a given week caused by influenza and pneumonia. The "epidemic threshold" is a certain percentage above what is considered normal for that period. The normal level, or baseline, is statistically determined based on data from past flu seasons.