Influenza is acquired by inhaling aerosolized respiratory secretions from a person with the disease. The virions attach by their hemagglutinin to specific receptors on ciliated epithelial cells, their envelope fuses with the cell membrane, and the virus enters the cell. Host cell protein and nucleic acid synthesis cease, and rapid synthesis of viral nucleoproteins begins. Regions of the cell membrane become embedded with viral glycoproteins, specifically hemagglutinin and neuraminidase. Within 6 hours, mature virions bud from the host cell, receiving an envelope of cell membrane containing viral hemagglutinin and neuraminidase as they are released. The virus spreads rapidly to nearby cells, including mucus-secreting cells and cells of the alveoli. Infected cells ultimately die and slough off, thus destroying the mucociliary escalator and severely impairing one of the body's major defenses against infection. The immune response quickly controls the infection in the vast majority of cases, although complete recovery of the respiratory epithelium may take 2 months or more. Usually, only a small percentage of people with influenza die, but even so, epidemics are so widespread that the total number of deaths is high. Influenza virus infection alone can kill apparently normal, healthy people. More often, however, death occurs because of bacterial secondary infections, usually by Staphylococcus aureus, Streptococcus pyogenes, or Haemophilus influenzae. Influenza takes a heavy toll on people whose hearts or lungs are weak.
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