Legionnaires' disease is a type of pneumonia caused by the bacteria legionella pneumophila. Ironically, action taken to close buildings to control coronavirus may in turn cause an increase in cases of Legionnaires' disease, an illness with striking similarities to COVID-19. This disease is spread when tiny droplets of infected water are inhaled, e.g. from showers or splashing taps. Normally the risk of legionella bacteria growing in water systems is minimised by ensuring that hot water is hot, cold water is cold and by preventing stagnation in the system. Shower heads and spray taps should also be cleaned regularly just in case these higher risk outlets become contaminated. More complex systems will have different and more detailed control measures.
The problem is that when buildings are closed up this inadvertently creates conditions which are more favourable to bacterial growth. In particular water temperature might reach 20°C to 45°C in some parts of the system, especially if water heating is left on or if there is a spell of warm weather. Plus water is likely to be stored for a long period rather than replenished daily with fresh water. The situation will be made worse if you already have conditions which promote bacterial growth such as sludge, scale or rust in the system. Regardless of this, stagnation can cause a biofilm to form on internal surfaces like taps, shower heads and hoses. Biofilms offer a nutritional source for legionella as well as create a protection from disinfection.
What to do during the shutdown?
These measures might not always be possible depending on your own circumstances or the Government lockdown rules however, they are the ideal precautions for small hot and cold water systems:
Draining down the water system isn't advised for short periods as it may actually promote the growth of bacteria in the system.
After a temporary period of little or no use: (1) dismantle and clean shower heads, hoses and spray taps; (2) heat water up to a high temperature and draw it through every outlet at a slow rate. This will thermally disinfect the pipes and outlets.
If your system is on a larger scale, seek advice from your water hygiene contractor on the required measures such as chemical disinfection of the cold water tanks and entire systems. They may also advise taking microbial samples a few days after recommissioning.