I met with Surrey University colleagues today. We may do a history of water sanitation, current issues and why they still exist. This example was given:
Giardisis (gastrointestinal infection) caused by giardia (commonly lamblia) in surface water in many parts of the world. It is treatable but is excreted back into the environment.
It can be filtered in towns, harder in rural areas. Is sensitive to chlorine, it involves different steps in sanitation process with two filtering steps, disinfectant.
Water companies know risks and put in appropriate treatment where possible but supply of equipment, disinfectant, when tests are completed (e.g. after rainfall, at night) can all affect the analysis.
This is also well summarised in this water research post: http://water-research.net/index.php/reports/giardia-crytosporidium-and-waterborne-disease