Pollution caused by human defecation in the open:
In villages or underdeveloped areas of the country people generally defecate in open fields due to the unavailability of proper toilets and poverty.
When this excreta is washed away by water due to rainfall for surface runoff, this excreta enters the rivers and streams and pollute it making the water unfit for drinking.
This polluted water if consumed by humans may cause diseases like cholera, diarrhea, typhoid etc.