influenza

a highly infectious disease caused by a VIRUS that affects the RESPIRATORY TRACT . Symptoms include headache, weakness and fever, appetite loss and general aches and pains. Sometimes there is the complication of a lung infection, which requires immediate treatment. There are three main strains of influenza virus, designated A,B and C. The viruses quickly produce new strains, which is why an attack of one is unlikely to provide protection against a later bout of the disease. Epidemics occur periodically; in Britain virus A is responsible for the majority of outbreaks.