Celebration wasn’t the only thing in the air in Philadelphia in July of 1976. Over the course of several days during the 58th Annual Convention of the American Legion, a killer mist spewed out of the air conditioning units throughout the building and into the sidewalks nearby. The result was a large outbreak of unexplained febrile pneumonia, often fatal, that would acquire the name Legionnaires’ Disease. What was causing this terrifying disease and how could it be stopped? In this episode, we walk through the massive investigation into this outbreak that would lead to the discovery of the causative agent, Legionella pneumophila, and explore the biology of this mysterious pathogen. We wrap up the episode with a look at the current status of Legionnaires’ Disease and a potentially grim forecast for its prevalence as the world slowly gets back to normal.