Legionella. It is a potentially fatal bacterium that for the most part has managed to fly under the radar of the general public since it was first determined as the cause of Legionnaires’ disease in the 1970s. Despite the fact that there are well-known techniques to prevent the growth of the causative bacteria, legionellosis (commonly known as Legionnaires’ disease) has taken the lives of well over 100,000 people in the U.S. alone. Furthermore, reported cases of the bacteria has risen 217 percent in just the last decade, according to the Centers for Disease Control and Prevention.