What is flu?
Flu, short for influenza, is a virus that is easily spread by the coughs and sneezes of people who have flu.
Flu is more common in autumn and winter - from October to early spring - which is why it is called seasonal flu.
Flu viruses attack the body though the respiratory tract.
It can affect people of any age - but it can be far more serious for older people, people with some medical conditions and during pregnancy.
Flu symptoms can come on quickly, and can include:
How long does flu last?
After you catch flu, symptoms usually begin a day to 3 days later. After experiencing a week of symptoms, most otherwise healthy people will start to feel better - but symptoms like coughing and feeling tired can last for some weeks longer.
Flu can last longer in people with weakened immune systems. It can also lead to death for hundreds of people each year in the UK.
What are the complications of flu?
Flu can lead to complications, including:
Flu complications are more common in young children, elderly people, pregnant women, people with long-term medical conditions, including diabetes, heart disease, cancer, lung disease and HIV.
What's the treatment for flu?
For most otherwise healthy people, flu can be treated at home with rest, plenty of fluids and over-the-counter painkillers, without seeing a GP.
For some people at risk of flu complications, antiviral medication oseltamivir (Tamiflu) or zanamivir (Relenza) may be recommended. These don't cure flu - but taken soon enough after symptoms appear, they can shorten the length of flu.
Antibiotics don't work for flu - but they may be recommended for complications of flu, such as a chest infection.
What's the difference between a cold and flu?
Colds and flu are both viral infections caught from coughs and sneezes.
Flu is usually more severe - stopping you from doing usual things - like going to work or school. Colds are usually milder and don't affect daily life as much.
Colds start more gradually than flu.
With flu, you'll usually have a fever and muscle aches, while colds tend to stick to the throat and nose.
There is an old saying that if you have flu and see a £50 note outside, you're too unwell to go out and grab it. Whether that's true or not - it gives an idea of how flu can hit you hard.