Speaking at a recent IHEEM seminar focusing on some of the key water hygiene and safety, and waterborne infection prevention issues, facing healthcare estates/ engineering personnel responsible for 'large, complex' water systems, Dr Nick Hill, technical director of the Water Hygiene Centre, examined some of the continuing areas of uncertainty over the characteristics and properties of different Legionella serogroups, the colonisation levels necessary to cause illness in individuals of varying susceptibility, potential sources, and the materials most likely to harbour the bacterium, and discussed effective methods for killing the organism, and preventing its spread. HEJ editor, Jonathan Baillie, reports.