What is the Flu?
The flu is a highly contagious viral infection of the upper respiratory system caused by the influenza virus. Found all over the world, the influenza virus causes yearly epidemics. Two types, A and B, are responsible for human infections—and within each type, there are different strains. This virus is so effective at causing illness because it has the capacity to mutate its genetic material often, so strains are different from season to season and worldwide. Immunity to one strain isn't perfect against another, and immunity to type A doesn't protect against type B, and vice versa.