1 Answer

     Influenza A is capable of infecting birds and swine, and these animals transmit it to humans,  This cross-species activity provides it with a lot of opportunities to mutate and become more virulent; it's the strain responsible for pandemics like swine flu.

     Influenza B is less common, and almost only infects humans (although dogs and ferrets can be infected).  This provides it with less opportunity to mutate, and this allows humans to build up some immunity to it, so it does not cause pandemics.

     Infuenza C is the least common strain, infecting dogs, pigs, and humans.  Although it can cause severe disease and local epidemics, it usually only causes mild disease in children.  I work in a hospital, where we test for Infuenza A and B, but I've never even heard of Influenza C, so that's how rare it is.