When you have touched things that lots of other people have touched!
Washing your hands is very important after you have been in a busy place (e.g. at work, school, shops, on public transport). If there are lots of people around it is very likely that surfaces will have germs on.
Viruses live longer on hard surfaces (e.g. benches, doors, rails, handles, money, mobile phones and keyboards) which makes them easier to pick up.
When you come in from outside.
You don’t want to spread germs you have picked up outside around your house. This is why it is a good idea to wash your hands when you come in or out of a building.
When you have been close to someone who is infectious.
Sometimes we cannot avoid being close to someone who is ill - especially children. To cut down your chances of catching their illness wash your hands as soon as you can after coming into contact with things they have touched.