Influenza (flu) is a very common, highly infectious disease caused by a virus. It can be very dangerous, causing serious complications and death, especially for people in risk groups. In rare cases flu can kill people who are otherwise healthy. 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). Flu leads to hundreds of thousands of GP visits and tens of thousands of hospital stays a year.
The flu virus is very variable and changes over time. Each year there are different strains around, and a new vaccine has to be prepared to deal with them. Vaccination from previous years is not likely to protect people against current strains of flu.
There are three basic types of flu: A, B and C. Type A is the most dangerous; it is the one that can cause serious disease and also triggers worldwide pandemics. Type C causes mild disease. Type B can make you feel very ill, but it has never led to a worldwide pandemic.
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 killed 40-50 million people worldwide. The World Health Organization (WHO) estimates that flu kills between 250,000 and 500,000 people around the world 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.