The flu is caused by a virus. It’s different to the ones that cause the common cold and spreads when someone coughs up or sneezes out tiny droplets that contain the virus.
You can catch it from someone sneezing next to you on the bus, by kissing someone who has the virus, or by touching something someone else who has the virus has touched. It then enters your body through your mouth, nose or eyes.
The germs can live on your hands and other surfaces for 24 hours, which is what makes it so contagious.