Legionnaires’ disease is becoming more common in Switzerland: 582 cases were recorded by the Federal Office of Public Health (FOPH) in 2019 – twice as many as in 2013. The disease is fatal in five to ten per cent of cases, despite treatment with antibiotics.

The causative agent is Legionella pneumophila, a type of bacteria found in water. Problems may arise if contaminated fine water droplets are inhaled. This can occur wherever such droplets are generated – in car wash facilities, air-conditioning cooling systems or industrial cooling towers. Under certain conditions, however, legionella can also develop in drinking water, fountains, steam baths and showers.

But how do these pathogens enter building plumbing systems? Most bacteria and nutrients are removed at the central treatment plants which provide municipal water supplies. Concentrations also remain low in distribution networks – the pipes which convey drinking water to buildings. “But then things become more problematic,” says Frederik Hammes of the Environmental Microbiology department at Eawag: water is heated in buildings, and as legionella bacteria grow best at temperatures between 35 and 40°C, they can colonise hot-water systems. Although legionella can be killed by heating water to 60°C, this temperature is rarely attained in showerheads, for example. In addition, in order to save energy, boiler temperatures in many buildings are set below 60°C. Legionella can, however, also proliferate in cold-water pipes if the water regularly reaches warmer temperatures. Other factors known to promote proliferation are nutrients leaching from plumbing materials and inadequate exchange of water (stagnation). But many questions remain to be resolved – for example, how precisely the occurrence of legionella in the environment is related to human infection, or how legionella can be most effectively detected in drinking water pipes.