Disinfection is a process where a significant percentage of pathogenic organisms are killed or controlled.  Since individual pathogenic organism can be difficult to detect in a large volume of water, disinfection efficacy is most often measured using "indicator organisms" that coexist in high quantities where pathogens are present.  The most common indicator organism used in the evaluation of wastewater is fecal coliform.  The most common indicator organism used in the evaluation of drinking water is Total Coliform.