Answer: Sewage
Explanation:
The leading cause of contamination in the countries without having water treatment plants is sewage. The sewage disposal is the main waste that needs to be treated.
Sewage has many harmful bacteria growing on it. This can be unhealthy if people are exposed to it. Sometimes the pipes leakage can take place infecting the nearby water bodies.
This can be a very serious issue affecting the freshwater which is used by the people for many domestic and commercial purposes.