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. There are two main types that cause infection: influenza A and influenza B. Influenza A usually causes a more severe illness. 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.