The Orthomyxoviridae (orthos, Greek for "straight"; myxa, Greek for "mucus") are a family of RNA viruses. They include five genera: Influenzavirus A, Influenzavirus B, Influenzavirus C, Thogotovirus and Isavirus. The first three genera contain viruses that cause influenza in vertebrates, including birds (see also avian influenza), humans, and other mammals. Isaviruses infect salmon; thogotoviruses infect vertebrates and invertebrates, such as mosquitoes and sea lice.

The three genera of Influenzavirus can be told apart by the structure of their proteins. They infect vertebrates, as follows:
 Influenzavirus A cause of all flu pandemics and infect humans, other mammals and birds
 Influenzavirus B infect humans and seals
 Influenzavirus C infect humans and pigs

Types 

There are three genera of influenza virus: Influenzavirus A, Influenzavirus B and Influenzavirus C. Each genus includes only one species, or type: Influenza A virus, Influenza B virus, and Influenza C virus, respectively. Influenza A and C infect multiple species, while influenza B almost exclusively infects humans.

Influenza A 

Influenza A viruses are further classified, based on the viral surface proteins hemagglutinin (HA or H) and neuraminidase (NA or N). Eighteen H subtypes (or serotypes) and eleven N subtypes of influenza A virus have been identified.

Further variation exists; thus, specific influenza strain isolates are identified by a standard nomenclature specifying virus type, geographical location where first isolated, sequential number of isolation, year of isolation, and HA and NA subtype.

Examples of the nomenclature are: 
 A/Moscow/10/99 (H3N2)
 B/Hong Kong/330/2001

The type A viruses are the most virulent human pathogens among the three influenza types and causes the most severe disease. 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.
 H1N2 is endemic in humans and pigs.
 H9N2, H7N2, H7N3, H10N7.

Influenza B 

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. This type of influenza mutates at a rate 2-3 times lower than type A and consequently is less genetically diverse, with only one influenza B serotype. 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. 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.

Influenza C 

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

References

Other websites 

 BioHealthBase Bioinformatics Resource Center Database of influenza genomic sequences and related information.

Viruses