In a developed nation with readily available drinking water, we take for granted our accessibility to potable water. Though with or without a water softening or filtration system it is highly unlikely you would ever encounter any problem with dangerous pathogens or disease, it is always helpful to be aware of what could be lurking. The most common waterborne disease-causing agents, from largest to smallest, are as follows.
Protozoa
Protozoa are single-cell parasites that range between 1 to 20 microns. Capable of surviving weeks, even months, in cold water, individual protozoa are capable of causing infection 2 days-few weeks following ingestion. This could lead to profuse diarrhea, vomiting, gas and intestinal discomfort lasting anywhere from 1 to 6 weeks.
Bacteria
With bacterial infection, ranging in size from 0.1 to 10 microns, infected people begin to experience symptoms within a few days to a few weeks. If experiencing the effects of bacteria ingestion, you could face diarrhea and potentially prolonged intestinal discomfort. In some countries around the world, even cholera can be the direct result from ingesting bacteria.
Viruses
With possible results such as hepatitis-A, rotavirus, enterovirus, norovirus, viral presence in drinking water can be particularly dangerous. With symptoms appearing anywhere from 1 day to several weeks, you could suffer not only diarrhea and intestinal discomfort but also a variety of other ailments.