Legionellae are bacteria that can develop in natural bodies of water such as lakes, ponds and rivers, but also in swimming pools and water pipes. They're dangerous when they reach a certain concentration. Legionella bacteria thrive in water between 25°C and 45°C, the optimum temperature for them to multiply.
Warning signs are fever, coughing, muscle pains, headaches and loss of appetite, often beginning two to ten days after exposure. After its onset the disease progresses in the form of slight to severe pneumonia, which in the very worst case can be fatal. Legionnaires' disease is treated with antibiotics.
The Legionella bacteria are absorbed via contaminated water droplets. This can easily occur in the course of everyday activities such as taking a shower, brushing your teeth or doing the washing up. Most at risk are people who smoke, who have a pre-existing lung condition such as asthma or COPD, or whose immune system is weak (for example owing to medication or therapy). According to the FOPH (Swiss Federal Office of Public Health), person-to-person transmission is possible but very rare. In 2017 the FOPH registered around 490 cases of Legionnaires' disease. In 2018, 294 cases had already come to light by mid-July.
There are already rules laying down how public establishments such as swimming pools and hospitals handle the supply of water to prevent the bacteria. So far, however, there are no binding guidelines for private households. The FOPH advises making sure that your hot water is running at a constant 60°C when it leaves the boiler and 55°C in the pipe system. The temperature of cold water should not exceed 25°C. While Legionella bacteria can survive at temperatures below this, they can't multiply. If water pipes haven't been used for some time it's also advisable to let them flush through thoroughly before using them again to get rid of any contaminated water.