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. If there are lots of people around it is very likely that surfaces will have germs on.
e.g. at work, school, cafes, shops, on public transport.
Viruses live longer on hard surfaces which makes them easier to pick up.
e.g. benches, doors, rails, handles, money, mobile phones and keyboards.