Influenza, or ‘the flu’, is a very common and highly infectious disease caused by a virus. It leads to hundreds of thousands of GP visits and more than ten thousand hospital stays each year.
It can be very dangerous and cause serious complications, sometimes even death. This is more common for some people in at-risk groups, for example, babies under six months, older people, and those with certain long-term medical conditions.
In the UK it is estimated that an average of 600 people a year die from complications of flu. In some years it is estimated that this can rise to over 10,000 deaths (see for example this UK study from 2013 , which estimated over 13,000 deaths resulting from flu in 2008-09).
The flu virus constantly changes, so different strains circulate yearly. The World Health Organization monitors the virus throughout the world and advises which 3 or 4 strains should be covered by an annual flu vaccine. Vaccination from previous years is not likely to protect people against current strains of flu.
Flu epidemics can kill thousands or even millions of people. The 1918 flu epidemic is estimated to have affected half the world's population and 40-50 million people died worldwide. The World Health Organization (WHO) estimates that between 250,000 and 500,000 people around the world die from the flu every year.
In the UK (and in the rest of the northern hemisphere) the annual flu season runs from about October to March or April. Most cases of flu occur between December and February.