The flu is an infectious respiratory illness. It is caused by the influenza virus, which circulates regularly, mainly during the winter months i.e. between December and March. A wave of the flu is practically part of a normal winter.

A sudden fever (> 38°C), coupled with general symptoms of illness, is a classic sign of influenza, as are chills, a runny nose, cough, sore throat, pain when swallowing, headache, dizziness as well as chest, joint and muscle pain. The symptoms can last one to two weeks. Children may also experience nausea, vomiting and diarrhoea; the elderly may have the flu but no fever.

Influenza is often confused with the common cold, but is differentiated by its sudden onset and frequently more serious symptoms.