The flu virus is contained in the millions of tiny droplets that come out of the nose and mouth when someone coughs or sneezes.
These droplets typically spread about one metre. They hang suspended in the air for a while before landing on surfaces, where the virus can survive for up to 24 hours.
Anyone who breathes in the droplets can catch flu. And anyone who touches the surfaces the droplets have landed on can also catch flu if they pick up the virus on their hands and then touch their nose or mouth.
Everyday items at home and in public places can easily become contaminated with traces of flu virus including food, door handles, the remote control, handrails, telephone handsets and computer keyboards.
So, it's very important to wash your hands frequently to prevent catching and spreading flu.
Read more about how to prevent the spread of flu.
New types of flu
If you become infected with a flu virus your body will produce antibodies against it. Antibodies are proteins that recognise and fight off germs that have invaded your body.
Your antibodies will remember this flu virus and fight it if it invades your body again.
But, over time the flu virus can change into a different version or strain, which means your body may not recognise it and you can catch flu again.
When the virus changes to a new strain that people have little or no resistance to, it can cause a flu pandemic, which means it can spread globally. This is what happened in the swine flu pandemic of 2009.