There are 4 types of influenza, but only types A and B cause seasonal epidemics every year; and are what is targeted by the flu vaccine. Type C can infect huma

There are 4 types of influenza, but only types A and B cause seasonal epidemics every year; and are what is targeted by the flu vaccine. Type C can infect humans but is typically mild and has not yet caused a serious epidemic. Type D is not known to infect humans at all.