While ground water generally contains bacteria, it should not contain any fecal coliform. When fecal coliforms are found, it suggests surface water or some waste source has directly entered the water supply. Since surface water receives frequent inputs from many sources, fecal coliforms are common and indicate recent contamination. The presence of coliform bacteria signals the possibility of disease-causing bacteria or virus in the water. Certain fecal coliform bacteria are good indicators of pathogenic microbes since they can be identified and measured much easier than more harmful pathogens.
Pathogens occur at very low levels, making testing difficult and very expensive.
When water supplies contain coliform bacteria in levels greater than one per 100 ml of water, the water may also contain pathogens that cause acute intestinal infections. While generally considered to be a discomfort to health, these infections can prove fatal for infants, the elderly and those who are ill. Water-related disease outbreaks tend to occur in the summer. Although rarely encountered today, typhoid, hepatitis and cholera, can spread through water supplies. Other sources, besides water, also transmit these diseases.