Indicator organisms indicate the presence of conditions which potentially can cause illness in people. It has been established through the EPA that certain levels of indicator organisms in recreational bathing waters represent a threat to public health. E. Coli is the indicator organism used in freshwater beaches, such as Lake Massapoag. Indicator organisms alone may not be harmful. Rather, since these organisms are found in the intestines of warm blooded animals (and people) the presence of these indicator organisms suggests that other harmful viruses and organisms (referred to as pathogens) may be present. If swimmers accidentally swallow water with these pathogens, recreational water illnesses may occur. The most common illness is gastroenteritis, which can be problematic to immunocompromised people.
Natural waters are prone to multiple sources of potential contamination—failing septic systems, waste from dogs, birds and other animals, very young children in diapers, commercial and agricultural drainage etc. Major rainfall events can also wash waste materials from the beach area and surroundings directly into the lake or other water bodies that feed into the lake. In many cases the problems are transient; the bacterial levels decrease rapidly within a day or even a few hours. Other conditions however may seriously harm lake water quality and potentially cause disease in persons exposed to swimming water contaminants.