Legionnaires’ disease (legionellosis) is an infection of the lung (pneumonia) caused by the Legionella
bacteria. Exposure to the Legionella bacteria will not necessarily lead to the disease, but the bacteria can cause a type of pneumonia that can be fatal. It can take 2 to 10 days for the symptoms to develop after inhaling the bacteria however symptoms usually appear within 5 to 6 days.