Influenza – commonly known as the 'flu' – is a serious infectious disease caused by influenza A and B viruses.
Influenza is usually transmitted within droplets shed during sneezing or coughing. These droplets are inhaled by another person, whereupon the virus infects the cells of that person's respiratory tract.

Influenza viruses rapidly destroy the mucous membrane of the respiratory tract (from the nose through to the bronchi). The main symptoms are abrupt onset of fever accompanied by severe headache and joint pains.
In many cases, bacteria then proliferate on the damaged mucous membrane, causing secondary infections such as nasal sinusitis, pharyngitis, middle ear infections and bronchitis.