Influenza (also called the “flu” or “seasonal influenza”) is a highly contagious acute viral infection caused by influenza viruses. Influenza viruses are part of the orthomyxoviridae family of viruses. While influenza viruses can be classified generically as belonging to a certain family (influenza type A, B or C), within a species unique serotypes are identified with letter/number combinations, such as H1N1, H3N2, etc, which is often how they are referred to in the media.
Each year, influenza illness can be caused by a number of different strains, but generally one strain dominates and causes the majority of the illness as in the 2009 pandemic year when H1N1 dominated. Influenza vaccines are changed each year to reflect the strains found to be causing illness. Because the influenza risk is present every year, it is especially important to have plans in place for dealing with its consequences.