ABSTRACT
The fundamental assumption upon which the entire field of epidemiology is based is that diseases do not occur by chance, and they arc not randomly distributed in the population. If diseases are not randomly distributed in the population, then the distribution that they do assume must tell us something about how and why that disease process occurred. There were at least five previously studied unsolved outbreaks which have subsequently been shown to be caused by Legionella pneumophila. The investigations of these outbreaks either confirmed the observations made during the 1976 Philadelphia outbreak or provided new information about legionellosis. Previous outbreaks have confirmed that the mode of transmission is by the airborne route, that person-to-person transmission does not occur, and that the incubation period of the pneumonic form is approximately 1 to 11 days with a mean of 4 to 5 days.
