Abstract: 1. History and Disease

Legionella were first isolated and identified as part of an investigation of respiratory illness in attendees of an American Legion convention in Philadelphia in 1976.^1,2 This highly publicized investigation documented 239 cases and 34 deaths due to a previously unrecognized cause of pneumonia. It was later shown that this disease occurs when sufficient numbers of Legionella are aerosolized from colonized water sources and subsequently inhaled by a susceptible host.³ The bacteria are associated with 2 forms of respiratory illness, collectively called legionellosis.^4,5 Legionnaires’ disease is the pneumonic and more severe form, which may be fatal in 15% to 20% of cases. The other form, called Pontiac fever after the first documented outbreak at a health department in Pontiac, MI, is a less severe, self-limited illness.⁴ Hypotheses to explain how one species of bacteria leads to two disease outcomes include differences in host susceptibility and the inability of some Legionella types to multiply in human tissue (due to virulence, host range, or bacteria viability, for example).^6–8