Out of 4,000 convention attendees, a total of 221 cases (5.5%) were infected. By the time the outbreak ended, of the original 221 “pneumonia” cases, 34 deaths (15%) occurred as a result of exposure to this previously unidentified bacteria. The causative agent would later come to be known as Legionella pneumophila and was isolated with its own given genus.

This was not the first outbreak. Retrospective studies have shown a number of pneumonia outbreaks, which we now know were caused by Legionella. The earliest documented outbreak of Legionnaires' disease occurred in 1957 in Austin, Minn. 78 people, 46 of whom were employees at a local meat packing plant, were hospitalized with acute respiratory disease of unknown cause. Two of the patients died. In July and August 1964, at least 81 patients at St. Elizabeth Medical Center in Washington, D.C., developed pneumonia after being exposed to Legionella bacteria. 17 of the patients died as a result. Evidence suggests that the bacteria may have originated from extensive soil excavations that were being carried out on the campus of the hospital during the summer months.

Outbreaks of Legionnaires' disease receive significant media attention. However, this disease usually occurs as a single, isolated case not associated with any recognized outbreak. When outbreaks do occur, they are are usually recognized in the summer and early fall, but cases may occur year-round. About 5% to 30% of people who have Legionnaires' disease die.