Although most coliform bacteria are harmless, the presence of any coliforms in drinking water suggests the possible presence of disease-causing bacteria, parasites, or viruses. These human pathogens can cause symptoms such as diarrhea, cramps, nausea, and vomiting, which together comprise a general condition known as gastroenteritis. Gastroenteritis is not usually serious for a healthy person, but it can lead to more serious problems in people with weakened immune systems, such as very young, elderly, or immuno-compromised individuals ((http://www.epa.gov/sites/production/files/2015-10/documents/cryptosporidium-report.pdf). Bacterial contamination in drinking water is not only related to fecal contamination, since some organisms, like Legionella, are able to grow in piped water distribution systems. http://www.epa.gov/sites/production/files/2015-10/documents/legionella-report.pdf.
Additionally, one particular strain of E. coli bacteria, E. coli O157:H7, has been known to cause severe illness in humans. Information on E. coli O157:H7 is available at . https://safewater.zendesk.com/hc/en-us/articles/214282698-4-What-are-the-health-effects-of-E-coli-O157-H7-.