Water treatment plants in most of North America have a process that eliminates harmful waterborne bacteria and parasites that may be present in natural water. Ultraviolet light is used to kill organisms in the water.

Chemicals, such as chlorine, are also added to the water to kill micro-organisms, but the levels are not harmful to the overall health of humans. Your lymphatic and immune systems also use processes to remove harmful substances to prevent you from being ill. Only harmful invaders or dead cells are recognized and destroyed by your body. Water treatment plants have human operators and computers to determine water quality and intensity of treatment. How does the human body determine its response to invading substances?