Novel Influenza Viruses

Three viruses in the family Orthomyxoviridae cause influenza in humans: influenza virus types A and B, which are responsible for annual seasonal epidemics, and influenza virus C, which is rarely described. Influenza viruses are enveloped, and have a segmented, single-stranded RNA genome. Hemagglutinin (H) and neuraminidase (N) are envelope proteins responsible for the attachment of influenza virus to host cells, and the release of virus from host cells, respectively. The antigenic characteristics of these proteins are used to further characterize influenza A virus subtypes. Influenza A virus subtypes H1N1, H3N2, H2N2, and H1N2 have circulated, or are currently circulating widely among humans. New strains of influenza A and B viruses emerge as the result of point mutations in the H gene (antigenic drift), or in the case of influenza A virus, after reassortment of H and N sequences from two different subtypes (antigenic shift). These new strains are responsible for annual outbreaks and epidemics of influenza, and the emergence of new influenza A subtypes is often responsible for global pandemics.