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.
Influenza occurs most often in winter and usually peaks between December and March in the northern hemisphere.
Illnesses resembling influenza that occur in the summer are usually due to other viruses.
The influenza virus was first identified in 1933. There are two main types that cause infection: influenza A and influenza B. Influenza A usually causes a more severe illness than influenza B.
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.
For most people influenza infection is just a nasty experience, but for some it can lead to more serious illnesses.
The most common complications of influenza are bronchitis and secondary bacterial pneumonia. These illnesses may require treatment in hospital and can be life threatening especially in the elderly, asthmatics and those in poor health.
Public Health England carries out laboratory tests to identify which strains of flu are in circulation, coordinates information at the UK level and communicates this information to other health professionals and to the public. This information is used to guide the development of policies for protecting the UK population from influenza.
Public Health England works closely with colleagues in Health Protection Scotland, Public Health Agency Northern Ireland, Public Health Wales, the Royal College of General Practitioners, NHS Direct, the Department of Primary Care at Nottingham University and the Department of Health to conduct surveillance of flu activity in the UK and uptake of seasonal flu vaccine in England.