Key facts

There are around a billion cases of seasonal influenza annually, including 3-5 million cases of severe illness

It differs from pandemic flu in that it is caused by an already circulating flu virus, whereas pandemic flu is a new virus against which people have little or no immunity

It causes 290,000 to 650,000 deaths every year..

Flu is very infectious and can easily spread to other people, with people being more likely to give it to others in the first five days.

Overview

Seasonal influenza, or the flu, is a common respiratory illness caused by flu viruses. It’s widespread and most people recover without needing medical attention. However, it can be severe or even deadly in some cases. There are four different types of flu virus: A, B, C and D. A and B virus cause seasonal flu.

Flu A includes subtypes like A(H1N1) and A(H3N2), with A(H1N1) being the virus behind the 2009 swine flu pandemic

Signs and symptoms

Flu symptoms start suddenly and typically appear one to four days after infection.…