Water that makes you sick may not be a problem all the time. It is worse when there is very little water because of no rain. It is also very bad when there is too much rain. When there is not enough rain many animals like pigs and dogs use the same water as people. Sometimes they spoil the water with their wastes.
Some sicknesses get into the water just by the pigs or dogs or cattle walking in it.
When there is very little water it can stand in ponds or mud holes and pollution is not washed away. It can be very dangerous to drink or wash in. When there is too much rain, wastes from people and animals wash into the water supplies. Soil with pesticides and weed killers and trash with lead and other poisons can get into the water. Sometimes after a big storm there are dead animals in the water and if anybody drinks it they will get sick.