Two families belong to this one, the Paramyxovirus and the Orthomyxovirus. Influenza virus belong to the latter. It was only perhaps during in the 1930's when the etiologic agent was identified to be a virus, rather than a bacteria.. Influenza virus has five genera: InfluenzavirusA, InfluenzavirusB, InfluenzavirusC, Isavirus and Thogotovirus. The virus that causes influenza has three immunologic types: A, B and C. Influenza type a is known to have great antigenic changes, type B to a lesser degree. Type C appears to be the most stable antigenically among the three. Types a and C. affect multiply species, while Type B exclusively infects humans. (2008 the Flu)

Influenza virus a is the prototype of the Influenza virus, and the structures of other Influenza virus are almost the same.

Influenza virus are usually spherical, but can also be globular and about 100 nm in diameter, sheathed in a lipid bilayer that is studded with two integral proteins, the some 500 molecules of hemagglutinin (H) and some 100 molecules of neuraminidase (N) and within it some 3000 molecules of matrix protein and 8 pieces of RNA which is associated with many copies of a nucleoprotein, several molecules of the three subunits of its RNA polymerase and some "non-structural" protein molecules of uncertain function.

The eight RNA molecules include the HA genes which encodes for the hemagglutenin, NA genes for the neuraminidase, NP for nucleoprotein, M genes for two proteins, a matrix protein and an ion channel spanning the lipid bilayer, the NS genes that encodes 2 different non-structural proteins and one RNA molecule for each of the three subunits of the RNA polymerase. (2008 the Flu)

These viruses infect humans by invasion of the respiratory...

First, hemagglutinin molecules bind to carbohydrate on the glycoproteins of the respiratory epithelial cells that when detected, the virus then is engulfed by receptor mediated endocytosis that cause a drop in pH in the endosome (endocytic vesicle) produces a change in the structure of the viral hemagglutinin enabling it to fuse the viral membrane with the vesicle membrane. The contents of the virus are then exposed to the cytosol after which the RNA enters the nucleus of the host cell where replication is made. The replicated viruses then return to the cytosol where some become mRNA, which is the only free RNA to be translated into the fresh batch of proteins of virus particles. The new virus, when mature, buds off from the host's plasma membrane. Multiplication of virus occurs rapidly where new virus progeny can be produced within 8 hours. These then overcomes the body's immune response, thus causing cell destruction in the respiratory tract. (Brooks et. al. 1995)
Reference:

Brooks, GF, Butel, JS, Ornston, LN 1995, Jawetz, Melnick and Adelberg's Medical Microbiology 20th ed, Appleton and Lange, Norwalk.

2008 the Flu, Available from: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/I/Influenza.html[April 24, 2008].

2005 Influenza Viruses, Center for Disease and Control Prevention, Available at http://www.cdc.gov/flu/avian/gen-info/flu-viruses.htm,[April 24, 2008].

2008 Influenza, Wikipedia, the Free Encyclopedia, http://en.wikipedia.org/wiki/Influenza[April 24, 2008].

Reeder, L 2000, the Deadly Potential of Influenza, Post Nuke, http://www.bact.wisc.edu/Microtextbook/index.php?name=Sections&req=viewarticle&artid=192&allpages=1&theme=Printer[April24, 2008].