In the summer of 1976, when 2,000 people came to Philadelphia for a national convention of the American Legion, a strange illness gripped hundreds of attendees, and more than 30 people died. Symptoms included cough, shortness of breath, and fever. Was it a virus, bacteria, some sort of toxin? A major investigation was launched by the CDC — and the gathering in Philadelphia became forever tied to one of the greatest medical mysteries of our time.    Almost 50 years later, reports of Legionnaires' Disease are in the news again. Reporter Elana Gordon tells the story of how scientists — with some serious shoe-leather detective work and a scientific breakthrough — tracked down the origin of this mysterious airborne illness.  On this episode, we listen back to her reporting from 2016: Audio Time Capsule: The discovery of Legionnaires’ disease.