Prior to the advent of microscopy and discovery of microorganisms, types of classes of living beings were either plants or animals Microbes did not fit into either of the traditional classes. A third classification to accommodate these, protists came into being Many protists have one cell, but even the multicellular ones have all identical cells. Tissue regions generally recognized as protists include: * Algae * Fungi * Protozoa * Bacteria * Cladocera * Copepods Macro invertebrates such as the * Nematodes * Chironomids * Snails There is another group of microorganisms, types that are not visible under the microscope. So, protists can be classified as highe protists (which have a more highly organised cell or eucaryotic cell) and lower protists (which have a simple cellular structure o procaryotic cell). The eucaryotic cell is present in protozoa, fungi and most groups of algae. The procaryotic cell is the unit of structure in bacteri and blue-green algae. A virus has a still simpler structure (which can not be classified as a cell). These microorganisms, types that are commonly found in wastewater, carry diseases that are public health hazards. They als produce a lot of toxins. For example, one type of blue-green algae releases toxins. Another type is toxic if ingested. Adverse healt effects from drinking water thus affected are not common, but such algae are known to produce gastroenteritis. Wherever blu green algae are known to cause problems, they must be prevented and controlled.