2.1
Influenza is an acute infection of the respiratory tract caused by the influenza A and B viruses. The symptoms of influenza include fever, cough, sore throat, myalgia and headache. These symptoms are not specific to influenza, and can be caused by other viruses (such as respiratory syncytial virus) which can present as an 'influenza-like illness'. Diagnosis of influenza can only be confirmed by laboratory testing, although the probability that an influenza-like illness is caused by influenza is higher if influenza is known to be circulating and if a person has a high fever. The symptoms of influenza-like illness can be different in infants and children and may include fatigue, irritability, diarrhoea and vomiting. Influenza infection is usually self-limiting and lasts for 3 to 4 days, with some symptoms persisting for 1 to 2 weeks. The severity of the illness can vary from asymptomatic infection to life-threatening complications. The most common complications are secondary bacterial infections such as otitis media, pneumonia and bronchitis. In the UK, the average number of deaths attributed directly to influenza is approximately 600 in non-epidemic years and between 12,000 and 13,800 deaths in epidemic years.