In 1976, a mysterious and deadly flu-like illness plagued hundreds of individuals who had attended a meeting of the American Legion in Philadelphia. Of the 221 individuals who fell ill following the meeting, 34 died, prompting a large-scale investigation into the cause of the outbreak.
Several months later, scientists at the Centers for Disease Control and Prevention (CDC) identified the cause of the illness — a previously unknown bacterium, which they named Legionella. The investigation determined the bacteria had likely originated in the cooling tower effluent before being drawn into the building air intakes and spread throughout the ductwork.