Microbiological agents range from single cell organisms to small living things such as Bacteria, Total Coliform Bacteria, Parasites, Protozoans, Fungi, Molds, and Viruses. In many cases, there is virtually no warning sign of a microbiological problem until someone becomes ill or sick. This is why proactive monitoring and regular screening is strongly suggested. In most cases, these contaminants are regulated under the Primary Drinking Water Standards and they should be "absent" or "not present" in the water.
Waterborne Pathogens include E. Coli, Legionella, Shigella, E. Coli 0157:H7 (note that most E.Coli in the gut are not disease-causing; this particular variety is), Fecal Coliform, Campylobacter, and Salmonella. A recent development that has improved the ability to determine the presence of waterborne pathogens is the use of Polymerase Chain Reaction (PCR) and qPCR.
Protozoans include contaminants such as Giardia, Cryptosporidium, and Blastocystis hominis which can be disease-causing.
They are categorized as a primary contaminant because they can directly impact your health, but, in some cases, they may also be associated with aesthetic problems. They can be characterized as pathogenic or disease-causing or can serve as vectors that facilitate the spread of disease or other disorders, such as Autoimmune diseases. Common disease-causing agents include Total Coliform Bacteria, E. coli, Waterborne Pathogens, Giardia, Cryptosporidium, Legionella, Shigella, E. Coli 0157:H7, Campylobacter, and Salmonella. There are about “7.2 million annual cases of illness in the United States due to waterborne pathogens, according to a new CDC estimate. They lead to nearly 120,000 hospitalizations and more than 6600 deaths every year.” Source: Science, 370, 1382 (18 Dec 2020).
Other microorganisms, such as: Nuisance Bacteria, Pink Bacteria, Iron Bacteria, Slime Bacteria, Sulfate-Reducing Bacteria, and standard plate count (heterotrophic bacteria) are typically associated with nuisance or aesthetic related issues, but they can also be associated with Corrosion and the premature failure of water-related infrastructure and equipment and elevated levels of corrosion by-products, such as: Copper, Chromium, Lead, Zinc, and other metals.