It is caused by the influenza virus. There are several forms of the virus and each can change over time. Each year scientists try to work out which form of the influenza virus is most likely to be the cause of the year's epidemic. Many people think of influenza as 'the flu' and use this term for common illnesses like head colds.
Influenza is much more serious.
In Australia, influenza causes as many as 23,500 hospital admissions and 1,500 deaths a year. And it is not only elderly people who are affected by influenza. Australia a great deal in doctor visits, hospital stays and days off work. The symptoms are quite different to the common cold or other respiratory infections. These often begin suddenly and develop quickly.