The most dangerous form is pneumonic legionellosis (also known as Legionnaire’s disease), which can potentially be fatal.
Legionella bacteria are all around in the natural environment, particularly in slow-moving surface water within lakes, rivers and ponds and soils. They can become a health hazard when they settle in hot and cold water systems like showers and recirculating systems where water is heated and allowed to cool.
The bacteria readily multiply at temperatures between 20°C and 45°C when a nutrient source (such as rust, sediment, algae and other organic material) is present. An infection can happen when contaminated water droplets are inhaled. Everyone is susceptible to infection, the risk increasing with age and individuals with impaired immune systems, diabetes, lung, heart disease or other pre-existing conditions.
Employers, or those in control of premises (such as a landlord), have a legal duty to identify, assess and reduce the risks of exposure to legionella as any water system, with the right conditions, can be a source of the bacteria.