Legionella in operational healthcare water systems: a case-based approach to investigation, interpretation, and control.

Legionella species are environmental Gram-negative bacteria capable of colonising artificial water systems and causing Legionnaires' disease, a severe form of atypical pneumonia. Healthcare facilities are particularly vulnerable to transmission because complex building water systems can support bacterial growth and aerosol generation, exposing highly susceptible patient populations. This article uses a case-based scenario to illustrate the investigation of suspected healthcare-associated Legionnaires' disease on a haematology ward. The recognition of clustered pneumonia cases prompted multi-disciplinary investigation involving infection prevention and control teams, clinical microbiologists, and estates engineers. The article outlines the regulatory and technical frameworks that underpin Legionella risk management in healthcare facilities and the role of Water Safety Groups in co-ordinating environmental investigation. Key elements of laboratory diagnostics are described, as well as practical aspects of environmental sampling, including the use of preflush and postflush samples to determine whether contamination is localised to outlets or reflects colonisation of the wider water distribution system. Interpretation of quantitative results is discussed alongside common engineering findings such as inadequate temperature control, stagnation, and redundant pipework. The article also outlines immediate precautionary measures and longer-term remediation strategies used to control Legionella in hospital water systems. Effective prevention of healthcare-associated Legionnaires' disease requires close collaboration between infection prevention and control teams, microbiologists, and estates professionals to ensure safe design, monitoring, and management of healthcare water systems.