There are several problems that can endanger the quality of drinking water. A number of these problems are summed up here.

Someone can detect coliform bacteria in drinking water. Coliform bacteria are a group of microrganisms that are normally found in the intestinal tract of humans and other warm-blooded animals, and in surface water. When these organisms are detected in drinking water this suggests contamination from a subsurface source such as barnyard run-off. The presence of these bacteria indicates that disease-causing microrganisms, known as pathogens, may enter the drinking water supply in the same way if one does not take preventive action. Drinking water should be free from coliform.

Yeasts and viruses can also endanger the quality of drinking water. They are microbial contaminants that are usually found in surface water. Examples are Giardia and Cryptosporidium. Giardia is a single cell organism that causes gastrointestinal symptoms. Cryptosporidium is a parasite that is considered to be one of the most significant causes of diarrhoeal disease in humans. In individuals with a normal immune system the disease lasts for several days causing diarrhoea, vomiting, stomach cramps and fever. People with weakened immune systems can suffer from far worse symptoms, caused by cryptosporidium, such as cholera-like illnesses.

Nitrate in drinking water can cause cyanosis, a reduction of the oxygen carrying capacity of the blood. This is particularly dangerous to infants under six months of age.

Lead can enter the water supply as it leaches from copper pipelines. As the water streams through the pipes, small amounts of lead will dissolve in the water, so that it becomes contaminated. Lead is a toxic substance that can be quickly absorbed in the human systems, particularly those of small children. It causes lead poisoning.

Legionella is a bacterium that grows rapidly when water is maintained at a temperature between 30 and 40 degrees Centigrade for a longer period of time. This bacterium can be inhaled when water evaporates as it enters the human body with aerosols. The bacteria can cause a sort of flue, known as Pontiac fever, but it can also cause the more serious deathly illness known as legionellosis