Legionella Infections (Legionnaires' Disease) By Larry M Bush, MD, FACP, Charles E Schmidt College of Medicine, Florida Atlantic University; Maria T Vazquez-Pertejo, MD, FACP, Wellington Regional Medical Center Reviewed / Revised Apr 2022 | Modified Sep 2022 View Patient Education Symptoms and Signs Diagnosis Treatment Key Points More Information Topic Resources Legionnaire's Disease (Chest... Legionella pneumophila is a gram-negative bacillus that most often causes pneumonia with extrapulmonary features.