Orthomyxoviridae are a family of RNA viruses which, so far as is known, infect mainly vertebrates (Thogotovirus in ticks,
Isavirus in the sea louse). It includes those viruses which cause influenza.

There are three genera of influenza virus, identified by antigenic differences in their nucleoprotein and matrix protein:
Influenzavirus A are the cause of all flu pandemics and are known to infect humans, other mammals and birds (see also
avian influenza),
Influenzavirus B are known to infect humans and seals,
Influenzavirus C are known to infect humans and pigs.

Known flu pandemics [1]
Name of pandemic           Date             Deaths              Subtype involved  
Asiatic (Russian) Flu    1889-90       1 million                  possibly H2N2
Spanish Flu                  1918-20     40 million                                H1N1
Asian Flu                      1957-58      1 to 1.5 million                       H2N2
Hong Kong Flu             1968-69      0.75 to 1 million                      H3N2

There are three types of influenza virus: Influenzavirus A, Influenzavirus B or Influenzavirus C. Influenza A and C infect
multiple species, while influenza B almost exclusively infects humans.[2]

The type A viruses are the most virulent human pathogens among the three influenza types and causes the most
severe disease. The Influenza A virus can be subdivided into different serotypes based on the antibody response to
these viruses.[2] The serotypes that have been confirmed in humans, ordered by the number of known human
pandemic deaths, are:
H1N1 caused "Spanish Flu".
H2N2 caused "Asian Flu".
H3N2 caused "Hong Kong Flu".
H5N1 is a pandemic threat in 2006-7 flu season.
H7N7 has unusual zoonotic potential.[3]
H1N2 is endemic in humans and pigs.
H9N2, H7N2, H7N3, H10N7.

Influenza B virus is almost exclusively a human pathogen, and is less common than influenza A. The only other animal
known to be susceptible to influenza B infection is the seal.[4] This type of influenza mutates at a rate 2-3 times lower
than type A[5] and consequently is less genetically diverse, with only one influenza B serotype.[5] As a result of this
lack of antigenic diversity, a degree of immunity to influenza B is usually acquired at an early age. However, influenza B
mutates enough that lasting immunity is not possible.[6] This reduced rate of antigenic change, combined with its
limited host range (inhibiting cross species antigenic shift), ensures that pandemics of influenza B do not occur.[7]

The influenza C virus infects humans and pigs, and can cause severe illness and local epidemics.[8] However,
influenza C is less common than the other types and usually seems to cause mild disease in children