Dr. A. Melvin Ramsay followed many of those who fell chronically ill during this 1955-1958 epidemic period for up to 34 years, until he died in 1989. All of these epidemics occurred in the late summer and autumn, decreasing in winter, with a new small peak of new female cases at Christmas in the Northern Hemisphere. The numbers of new cases would then rapidly fall off as the winter months progressed, only to reappear in the late summer again. Over 60 similar but often less known epidemics have been documented by Dr. J. Gordon Parish. These epidemics have been associated with certain particular characteristics.
Onset can be at any season but most frequently occurs in summer or autumn.
There appears to be a high prevalence of epidemics or clusters in schools, hospitals and institutions involving hospital staff. Teachers, residential students or students engaged in team sports or orchestras traveling in groups appear more affected. The illness is less common in patients hospitalized for other causes and non-residential students. In patients seen at Nightingale, staff and teachers at institutional schools, particularly for the physically or mentally disabled appear to be greatly at risk. The usual incubation period of the triggering illness is 4-6 days.