The 2009 H1N1 virus, like other influenza A viruses, is an enveloped, single-stranded RNA virus. Influenza viruses are members of the family Orthomyxoviridae and are divided antigenically into 3 major types, A, B, and C. Influenza A viruses have the ability to undergo continuous antigenic changes, whereas Influenza B viruses reassort to a much lesser degree and influenza C viruses are antigenically stable.
Due to their ability to undergo antigenic changes, influenza A viruses can cause more infections with greater morbidity in the human population than influenza B or C viruses. Influenza A viruses are subtyped based on two glycoprotein spikes expressed on their surface:
1) Hemagglutinin (HA)
2) Neuraminidase (NA)
HA, a viral attachment protein, helps to facilitate the attaching of the virus onto ciliated epithelial cell receptors in the respiratory tract. NA is an enzyme that facilitates the release of virus particles from the infected cell surface.
