Pathogenic organisms (e.g., Norovirus, Cryptosporidium, Salmonella) are present in very low concentrations in most water samples and are difficult and expensive to detect individually. Fecal indicator organisms like E. coli are far more abundant when fecal contamination occurs, are reliably correlated with pathogen presence, and are much easier to measure. Regulatory frameworks (WHO, EU Drinking Water Directive 2020/2184) therefore use E. coli and Enterococci as proxy indicators for water safety.