The viruses that cause influenza in humans are classified into three types morphologically similar: A, B and C. Influenza viruses A, the most frequently studied are the most important for its ability to undergo antigenic changes, resulting in epidemics and pandemics and infect animal populations. A virus is based on two surface antigens: hemagglutinin (H) and neuraminidase (N), giving rise to this name is so familiar today in the media with the issue of avian and swine influenza. Full identification of the strain contains the place of origin, number of isolation, year of isolation, and subtype (eg. A/Victoria/3/79 H3N2 influenza virus).
Influenza viruses B and C are identified similarly, but the antigens M and N do not have subtype names because they have less variation.

Structurally, the virions are irregular spherical particles with a diameter of approximately 100 nm (80 to 120 nm), covered by a lipid membrane in the looming glycoproteins hemagglutinin (H) and neuraminidase (N).
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