Moving clean water is expensive So in a poor country there will be many people who are not connected to clean water.
In a poorer country people may have to go and get their own water from a river. This water will be warm and muddy. It will not be clean and will contain many germs which can cause illness, sometimes death. Even in cities, which are the easiest to provide for, there are many areas where there are few pipes, and these will only be standpipes - taps that people have to use in the street.
