The oocysts (the thick-walled stage of a one-celled protozoan parasite) of Cryptosporidium are very common and persistent in water, resistant to chlorine disinfection, and can cause massive outbreaks of disease as seen in Milwaukee, Wisconsin in 1993 when 400,000 people became sick from this parasite. Cryptosporidium can be transmitted from person to person, livestock to humans, through the oral-fecal route, by inanimate objects or by recreational waters. It causes profuse diarrhea, dehydration, fever, and weight loss especially in individuals who are immunocompromised, in renal failure, and have liver disease. It may cause an acute episode and also a chronic illness of recurring intermittent episodes. A low dose of the oocysts can cause the disease in healthy people.
Cyanotoxins which are produced by cyanobacteria blooms in freshwater are a health problem both to people worldwide and to the various ecosystems. The sharp increase of nutrients in water including phosphorus and nitrogen has caused eutrophication and this is favorable for the growth of large quantities of harmful algae blooms, especially when there is a high nutrient concentration and high light intensity. The toxins, microcystins, cylindrospermopsin and other cyanotoxins, can cause gastroenteritis, liver damage, and kidney damage.
The algae bloom can cause increased taste and odor problems; increased pH; increased turbidity; shortened filter runtimes; need for increased coagulant; and increased chlorine demand or decreased chlorine residual.