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Microbiology and Pathogenesis
Influenza A, B, and C viruses are RNA viruses and members of the family Orthomyxoviridae that have different nucleoprotein (NP) and matrix (M) protein antigens. Influenza A and B viruses are major human pathogens and are morphologically similar; influenza B infection is associated with less severe disease than influenza A infection, and influenza C virus causes subclinical disease.
- Influenza A viruses are subtyped by surface hemagglutinin (H) and neuraminidase (N) antigens.
- Virus attaches to sialic acid cell receptors via the hemagglutinin. Neuraminidase degrades the receptor and plays a role in the release of virus from infected cells after replication.
- Antibodies to the H antigen are the major determinants of immunity, while antibodies to the N antigen limit viral spread and contribute to reduction of the infection.
- Influenza is acquired from aerosolized respiratory secretions of acutely ill individuals and possibly by hand-to-hand contact or other personal or fomite contact. Viral shedding usually stops 2–5 days after disease onset.