Influenza viruses can be divided into three classes, A, B, and C, largely based upon conserved antigenic differences in the internal nucleoprotein.
Influenza A virus, typically encountered more frequently than types B and C, and associated with the majority of serious epidemics, can be further subdivided into strains or subtypes based on antigenic differences in the external hemagglutinin proteins (H1-H16) and neuraminidase proteins (N1-N9).
Influenza B virus is predominantly a human pathogen, although it has been found to infect seals. The limited host range of influenza B and a slower rate of mutation than influenza A appears to preclude development of influenza B pandemics, but influenza B is a significant human pathogen and on an individual basis, infection may result in death.
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