"Influenza A is an envelope virus containing a segmented negative RNA genome. The viral envelope has two major surface glycoproteins, Hemagglutinin (HA) and Neuraminidase (NA) of which there are 16 and 9 subtypes respectively. Influenza outbreaks usually occur in the winter in temperate climates, spread person to person via small particle aerosols. In the United States, the 'flu season usually starts in October or November and is at its height from December to March. Major outbreaks of influenza are associated with influenza virus type A or B. Infection with type B influenza is usually milder than type A. Type C virus is associated with minor symptoms. The disease is usually most severe in very young children (under 5 years of age) and the elderly. Young children often lack antibodies to the influenza virus because of no prior exposure. In addition, the small diameter of components of the respiratory tract in the very young also means that inflammation and swelling can lead to blockage of parts of respiratory tract, sinus system or Eustachian tubes. *VIROLOGY - CHAPTER THIRTEEN, INFLUENZA VIRUS (ORTHOMYXOVIRUS) by Dr. Margaret Hunt"