Legionnaires' Disease is a potentially fatal form of pneumonia caused by the inhalation of vapour from contaminated water sources that produce aerosols containing the bacteria. In a property the main areas of risk are where the bacteria can multiply and increase to harmful and potentially dangerous levels and then be spread through aerosols, e.g. in vapour and spray created by showers and taps.
As systems become more complex and subject to use, further considerations will apply to fully comply with the law.