Standing water in the tank and damp filters create a favorable environment for the proliferation of microorganisms. The bacterium Legionella poses a particular danger: it causes severe pneumonia known as Legionnaires' disease. This illness can be fatal for individuals with weakened immune systems. According to the doctor, it initially masquerades as the flu but quickly affects the lungs. Another threat is hypersensitive pneumonitis, or humidifier lung. This is an allergic reaction to inhaling mold and fungus spores that proliferate inside the device. Symptoms include a dry cough, shortness of breath, fever, and chronic fatigue.