Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Legionnaires' disease, commonly known as Legionellosis is a form of a severe, potentially fatal pneumonia, which can be caused by the legionella bacterium. The name Legionnaires' disease became known in the middle 1970s, when there was an outbreak of a strange form of pneumonia among delegates at a convention of the American Legion in Philadelphia, USA. Annually, statistics show that up to 18,000 people are hospitalized with Legionnaires' disease in the U.S. alone. Learn more about the Legionnaires infection in this book.