High concentrations in a water sample mean the water is contaminated with feces. Coliforms themselves aren't necessarily hazardous, but the presence of coliforms indicates that other hazardous microorganisms might be in the water. These microorganisms can cause what physicians call enteric diseases. Enteric disease microorganisms infect the cells of the intestinal lining, either growing on them or in them. The resulting diseases can be serious, and occasionally fatal.
Symptoms can include cramping, diarrhea, vomiting, nausea and sometimes fever.
Enteric disease microorganisms can be in the form of bacteria, viruses or protozoa. In the last several years these agents have contaminated city water supplies in several locations around the country. People killed by microorganisms in water are usually the very young, the very old, and those individuals with insufficient immune systems. Disease-causing protozoa can sometimes be found in water in rural or developing areas. Generally, wild animal feces containing the protozoa contaminate the water supply.