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![Spring Lake in Arizona showing algal blooms along the edge. (© Chris Keane, American Geological Institute) Spring Lake in Arizona showing algal blooms along the edge.](https://www.americangeosciences.org/sites/default/files/styles/ci__650_x_430_/public/ERN_water_algae_Keane_h70jur.jpg?itok=fOn7ZjhD)
Microorganisms that cause illnesses also affect water quality. Bacteria are single-celled organisms that cannot be seen with the human eye except through a microscope. Some kinds of bacteria are the most dangerous microorganisms. They usually get into water supplies when untreated sewage mixes with the water supply. It is not only a matter of human wastes from leaky sewer pipes. Dog and cat droppings are also deposited on land inside and outside the city.