Influenza ('Flu) is an illness caused by the influenza virus. There are different strains of influenza virus. They are transmitted by coughing and sneezing. Symptoms of influenza include a high temperature (fever), muscle aches, cough, headache and extreme tiredness. (Up to date links are at the bottom of the page).
Most people recover fully, but complications such as a chest infection or pneumonia develop in some cases. Complications are sometimes serious and can even be fatal in some cases. Complications are more likely to develop in the elderly and also in people with some underlying medical problems such as chest or heart conditions. Many people in the UK die each year (mostly elderly people) from the complications of influenza.
Each winter a different strain of the influenza virus causes an outbreak of influenza which affects many people. This is called seasonal 'flu. Vaccine is prepared by manufacturers under instruction from the Government as to which strains should be included in the vaccine (which itself is largely determined by strains circulating in the Southern Hemisphere during their winter season)
Go NHS Choices 'Flu Information for more information