Which of the following is the most likely way to get bacteria and viruses to enter a water supply

This is part of an essay I wrote on how Coliform bacteria enter water supplies ... Coliforms enter water supplies from the direct disposal of waste into streams or lakes, or from runoff from wooded areas, pastures, feedlots, septic tanks, and sewage plants into streams or groundwater. In addition, coliforms can enter an individual house via backflow of water from a contaminated source, carbon filters, or leaking well caps that allow dirt and dead organisms to fall into the water

Lack of proper sanitation facilities and water drainage system can cause merging of fresh water with the sewage and allow bacteria, viruses and various other microbes to enter the water supply. This causes infectious diseases.