Influenza is a syndrome that is caused by infection either influenza type A or B virus.
Other viral illnesses may mimic influenza, but only influenza occurs in epidemics that affect all age groups. Influenza virus strikes in yearly epidemics. Worldwide epidemics are referred to as pandemics. These epidemics begin each year in the orient, each caused by a new strain of virus. The virus strain is named for the location it was first isolated.