Legionella bacteria occur naturally in freshwater but can multiply in building water systems where temperature, stagnation, deposits and biofilm create favourable conditions. Legionnaires' disease is a serious pneumonia normally associated with breathing in contaminated water aerosols from systems such as showers, cooling towers and spray outlets. HSE guidance identifies water at around 20 to 45 degrees Celsius, nutrient sources and aerosol generation as factors to consider. Prevention depends on a proportionate assessment of the actual system, an accurate schematic, competent responsibility and an effective written scheme where risk requires control. Temperature is often the primary strategy for hot and cold water systems, supported by appropriate circulation, cleanliness and monitoring. Sampling can help investigate or verify control, but a negative result at one point does not prove the whole system safe. Chemical dosing or one-off disinfection does not replace correction of poor design, stagnation or failed temperature control. Managers should prioritise vulnerable occupants, document action on failed limits and review the assessment when use, plant or results change. Read the full guide for practical checks and sources from the Health and Safety Executive.