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Influenza or 'flu' is a respiratory illness associated with infection by influenza virus. Symptoms frequently include headache, fever, cough, sore throat, aching muscles and joints. There is a wide spectrum of severity of illness ranging from minor symptoms through to pneumonia and death.
The influenza virus is unstable and new strains and variants are constantly emerging, which is one of the reasons why the flu vaccine should be given each year. Influenza infection for most people is mildly dibilitating but in some cases can lead to serios illnesses eg bronchitis and secondary bacterial pneumonia which may require treatment in hospital and can be life threatening especially in the elderly, asthmatics and those in poor health.