Legionella naturally occur in ground and surface water and thrive best in water at temperatures between 25-45°C. When ingested orally, Legionella are harmless. However, the inhalation of water contaminated with Legionella via the respiratory tract (aerosol formation, steam, showers, air conditioning, etc.) can cause a Legionella infection. Symptoms range from flu-like illness to fatal pneumonia.  
The Drinking Water Ordinance (TrinkwV 2001) contains a technical action level. The obligation for sampling is regulated by §14 para. 3 in conjunction with Annex 4 Part II of the Drinking Water Ordinance.