Types

There are three types of Influenza virus: Influenzavirus A, Influenzavirus B, and Influenzavirus C. Influenza A and C infect multiple species, while Influenza B almost exclusively infects humans.

Influenza A

Influenza A is further classified, based upon the surface proteins hemagglutinin, and neurominidase; sixteen subtypes have been found to this date. 
The type A viruses are the most virulent human viruses among the 3 influenza viruses, and causes the most severe diseases. Here is a list of human pandemics caused by Influenza A:


H1N1 caused Spanish flu in 1918; Swine flu in 2009
H2N2 caused the Asian flu
H3N2 caused the Hong Kong flu
H5N1 is a pandemic threat
H1N2 is endemic in pigs and humans

Influenza B

Influenza B is almost exclusively a human virus, and is much less common than Influenza A. The only other known animal that is susceptible to Influenza B is the seal. Influenza B mutates at a rate about 2-3 times slower than Influenza A, therefore making it less deadly, and also causing there to be only on serotype. Also, a degree of immunity to Influenza B is acquired at a young age, but it does mutate fast enough that continued immunity is not possible. 

Influenza C

Influenza C is even less common than type B. It infects pigs and humans, and cause severe illness and local pandemics, but however seems to usually cause mild disease among children.