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The flu is an infection of your airways. It's caused by the flu virus. The flu is normally worse than a cold, and it can make you feel very sick.

Flu stands for influenza. There are three types of influenza virus: A, B and C. Types A and B can make you very sick.¹ Type A is the world's most common flu virus.² It usually makes you more ill than type B.³ Type C isn't normally very serious.

Doctors diagnose the flu by looking at your symptoms. They only test for a particular virus if they are monitoring a type of flu that could spread to lots of people.

The flu virus lives in your nose, your throat and the air passages leading to your lungs. You usually get a fever, body aches, chills and a dry cough. The flu virus can cause more serious problems, such as pneumonia.¹

When you're infected with a virus, your immune system tries to fight it. If you've recovered from a virus before, your body knows how to deal with it in future. This means that you won't get sick if you come into contact with the same virus again. But the influenza virus changes all the time,.¹ This is called mutation. So you can catch the flu even if you've had it the year before.

The flu spreads very easily from person to person. You can get it from people who cough, sneeze or talk near you.³ You can get the flu from someone while they are sick or a few days before they have any symptoms.

Some people in Asia, Russia and Turkey have caught a more serious type of flu from close contact with birds. It's called avian flu or bird flu. To read more, see Avian flu.

Flu vaccines help to prevent some types of flu. To read more, see Flu vaccines.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.