= Influenza A virus
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<Influenza A virus> is an enveloped <RNA> virus in <Orthomyxoviridae> with eight segments of <Negative-sense RNA>. Its surface proteins include hemagglutinin and neuraminidase, whose H and N labels identify familiar subtype designations such as H1N1. A subtype label is a classification of surface proteins, not a complete specification of every genome segment.<Orthomyxoviridae report by the International Committee on Taxonomy of Viruses>[Nucleic acid and influenza A distinguishing features]{ref}<Types of Influenza Viruses by the Centers for Disease Control and Prevention>[Influenza virus basics: H/N subtype notation]{ref}
Segmentation makes <reassortment> possible when compatible viruses contribute different segments to progeny. For an original schematic, label two parental segment sets $A_1,\ldots,A_8$ and $B_1,\ldots,B_8$. A progeny genome could contain $A_1,A_2,B_3,A_4,B_5,B_6,A_7,B_8$: whole segment identities have changed in combination without requiring a crossover inside a segment. This illustrates the mechanism, not a claim that all $2^8$ combinations are equally viable or occur equally often.<Types of Influenza Viruses by the Centers for Disease Control and Prevention>[Definition of segment reassortment]{ref}
The virus is also informative beyond classification and disease. Electron-microscopic studies of polarized epithelial cells observed apical influenza budding, contrasting with basolateral <vesicular stomatitis virus> budding. Thus particle assembly can report the spatial organization of a host <cell membrane>.<Asymmetric budding of viruses in epithelial monlayers: a model system for study of epithelial polarity by E. Rodriguez Boulan and D. D. Sabatini>[1978, asymmetric budding observations]{ref}
The eight-segment genome, a surface-protein subtype and a measured biological phenotype answer different questions. Genetic similarity alone does not guarantee identical host range, transmissibility or immune recognition.
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