Legionella is a bacterium that can cause pneumonia in its victims when they come into contact with a contaminated aerosol of water. It is a bacterium that multiplies in conditions where there is water stagnation or low flow, warm temperatures between 20-45°C, and a food source. All these conditions can occur when a water system is stagnant (no water movement). Water systems become stagnant for a number of reasons. These include buildings being mothballed, school holidays, and the period between system commissioning and hand over.