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.