Influenza is a respiratory illness associated with infection by Influenza viruses. Influenza type A is most common and causes the most serious epidemics and pandemics. Influenza type B may cause epidemics, but usually causes milder illness than type A. Influenza viruses are highly contagious and usually spread from person to person when an infected person coughs, sneezes, or talks and the virus is sent into the air. Persons in semi closed or crowded environments, such as students, prisoners and residents of nursing homes, are at high risk for exposure. Influenza proves fatal most oftenin newborns, the elderly, the immunocompromised, and the chronically ill. The small airways of infants and very young children make them susceptible to severe case of influenza. The most common complications of influenza are bronchitis and secondary bacterial pneumonia. Influenza and pneumonia together are the sixth leading cause of death in the United States and the fifth leading cause in older adults. Each year, Influenza causes approximately 20.000 deaths and there is an average of 114.000 Influenza-related hospitalisations.

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