An outbreak of a fulminant pneumonia occurred in 1976 with persons attending a state American Legion convention in Philadelphia, PA USA. The causative agent was identified as a bacterium, Legionella pneumophila. Outbreaks have been linked to cooling towers, evaporative towers, air-conditioning units, hot-tubs, charcoal filtered water, and dust at construction sites. Most recently, a case was linked to septic waste contaminated compost. The treatment of choice has been erythromycin. Since this is an air born infection, smokers and immunosuppressed patients are the most susceptible population.