Its full name is 'Influenza'. Lots of the time people say they have 'flu' when all they have is a cold and a bit of a temperature. Real 'flu' is caused by one of the many influenza viruses which infect your throat but make you feel unwell all over. These viruses give you a high fever, aching muscles, a headache, make you feel shivery and rotten and all you want to do is to lie down and fell ill. Sometimes you may get a cough. These problems usually last a 3-4 days.
There are a couple of different types of 'flu' - Seasonal and Pandemic ('swine flu'- check it out elsewhere on this site ).
This is the most common type of flu and occurs in yearly cycles, with most infections in the Winter. The influenza virus causing this form of influenza tends to change a bit from the year before and so most people have some natural resistance to it. You may still become a bit unwell for a few days from 'Seasonal Flu' but only very old people, babies and those with a serious lung disease (eg bad asthma, cystic fibrosis) are at risk from a serious infection. Every year they make a vaccine for 'Seasonal Flu' so if you have a bad lung disease speak to you GP in the Autumn about having it.
What to do
Lie down; take paracetamol as directed on the packet; get mum or dad to look after you; drink lots of clear fluids; stay away from anyone you like or you will spread it to them; hang on in there till you feel better - remembering it is all over in a couple of days or so! 'Flu' jabs are only recommended for older people or if you have some other serious illness for which you are receiving treatment.