Influenza is an infection caused by a virus called the influenza virus. It is usually a more severe illness than the common cold which is caused by other respiratory viruses.

Influenza typically has a sudden onset with headache, chills and cough followed rapidly by a fever, appetite loss, muscle aches, and tiredness. On the other hand, cold symptoms are limited to the upper respiratory tract with runny nose, sneezing, watery eyes, and throat irritation.

Gastrointestinal symptoms, such as nausea, vomiting or diarrhoea, can sometimes accompany influenza, especially in children, but these symptoms are rarely prominent. The term "stomach flu" is often incorrectly used to describe gastrointestinal illness that is due to other causes.

There are three main types of influenza:

a) influenza A,
b) influenza B, and
c) influenza C.

Influenza A and influenza B are associated with annual outbreaks and epidemics. Influenza A usually causes more severe illness than influenza B and can result in pneumonia, hospitalization or even death, especially in the elderly and those with chronic illnesses. Influenza A has also been responsible for all previous pandemics (worldwide epidemics of influenza).

Influenza C is very rarely seen and is associated with only mild sporadic illness.