... a genetic entity that exists somewhere between a living and non-living state. Viruses exist only to make more viruses, content to reproduce and kill off the healthy host cells they invade. The nasty influenza viruses are no different.
Influenza viruses come in types A, B, and C. Type C infection usually causes either a mild respiratory illness or no symptoms at all. The A and B viruses-which can cause influenza epidemics-continually undergo an antigenic drift, slowly mutating from year to year. Because of this, vaccines only work for the virus prevalent in a single season-and you end up needing a new flu shot each year.