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Influenza A virus and influenza B virus circulate in humans and cause seasonal epidemics, and influenza C virus causes a mild infection, primarily in children. Influenza D virus can infect humans but is not known to cause illness. In humans, influenza viruses are primarily transmitted through respiratory droplets from
Influenza hemagglutinin (HA) or haemagglutinin [p] (British English) is a homotrimeric glycoprotein found on the surface of influenza viruses and is integral to its infectivity. Hemagglutinin is a class I fusion protein, [1] [2] having multifunctional activity as both an attachment factor and membrane fusion protein.
Influenza A virus structure. The influenzavirus virion is pleomorphic; the viral envelope can occur in spherical and filamentous forms. In general, the virus's morphology is ellipsoidal with particles 100–120 nm in diameter, or filamentous with particles 80–100 nm in diameter and up to 20 μm long. [5]
There are four different types of influenza virus: A, B, C, and D. Influenza C usually causes only mild illness while D mostly affects animals, especially cattle.
The subtypes of influenza A virus are estimated to have diverged 2,000 years ago. Influenza viruses A and B are estimated to have diverged from a single ancestor around 4,000 years ago, while the ancestor of influenza viruses A and B and the ancestor of influenza virus C are estimated to have diverged from a common ancestor around 8,000 years ago.
There are various subtypes of hemagglutinins, in which H1, H2, and H3 are known to have human susceptibility. [8] It is the variation in hemagglutinin (and neuraminidase) subtypes that require health organizations (ex. WHO) to constantly update and surveil the known circulating flu viruses in human and animal populations (ex. H5N1).
Influenza viruses, like all viruses in the family Orthomyxoviridae, are enveloped RNA viruses with single stranded negative sense RNA genomes. [2] Divergent evolution of the matrix protein (M1) and nucleoprotein (NP), are used to determine if the virus is type A, B, C, or D. [4] The M1 protein is required for virus assembly and NP functions in transcription and replication.
To unambiguously describe a specific isolate of virus, researchers use the Influenza virus nomenclature, [21] which describes, among other things, the subtype, year, and place of collection. Some examples include: [22] A/Rio de Janeiro/62434/2021 (H3N2). [22] The starting A indicates that the virus is an influenza A virus.
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