Contaminated water supplies have been responsible for major outbreaks of severe gastrointestinal illnesses such as gastroenteritis and infections caused by the protozoan parasites Cryptosporidium and Giardia. Gastrointestinal illnesses can be particularly severe for the very young, the elderly and people with weakened immune systems.Private water supplies can also be contaminated by various chemicals or metals which, at elevated levels, can be harmful to your health.