An analysis of statistics compiled on outbreaks of waterborne disease from 1946 to 1980 and from 1965 to 1982 showed that disinfection was not provided where it was needed and was not properly operated where it was provided. This article reviews the factors that influence chlorine's effectiveness in inactivating bacteria, viruses, and protozoans for the purpose of preventing and controlling waterborne disease outbreaks. The factors reviewed are chlorine concentration, contact time, pH, temperature, and interfering (chemical and physical) substances. Each of the factors are evaluated to determine disinfection capability under operating conditions and to determine the adjustments necessary for coping with the challenges of pathogens. Includes 15 references, tables, figure.