The usage of water with poor microbiological quality increases the risk of human illness. This review discusses and updates current thinking on the nature of the interaction between a range of human bacterial pathogens and waterborne protozoa. The importance of protozoa acting as protective environments for pathogenic bacteria from disinfection and of promoting extended survival in otherwise hostile environments is highlighted. The significance of biofilms in water systems, and new relationships between Salmonella and Campylobacter and water-borne protozoa are also discussed. The protection of pathogenic bacteria from disinfection within protozoa and/or biofilms has important implications for water safety. (c) 2005 Elsevier SAS. All rights reserved.