Legionnaires’ disease is a form of pneumonia caused by Legionella bacteria. It is an infection that can be fatal if not properly treated. According to the Centers for Disease Control and Prevention, after researchers analyzed cases in New York City and 20 other jurisdictions, the bacterium linked to Legionnaires’ disease was found in a number of health care facilities. In fact, of the jurisdictions they studied, they found 16 of them had some cases that came directly from hospitals.