The search for the cause of the large outbreak of severe pneumonia that affected attendees of a convention of war veterans in Philadelphia in 1976 was complicated by a number of factors. The 2–10 day incubation period meant that cases were dispersed across Pennsylvania at time of onset, and matters were further complicated by the lack of a centralised record of attendance of the convention. Known methods for culture, serology, tissue staining, and toxicology gave negative results. The reliance on a clinical case definition, and the need to coordinate local, state, and federal jurisdictions both hindered efforts. Intense public scrutiny, anxiety, and promotion of implausible theories, researchers’ memories of prior unsolved major investigations, and the inherent difficulty of documenting airborne spread also had an impact. The novel bacterial agent Legionella pneumophila was identified and its spread defined by persistent application of basic epidemiological and laboratory principles, including the willingness to re-examine one’s assumptions.