Over a period of years, physicians in the rural southeastern United States reported the occurrence of a disease that had not been recognized previously. Since reporting was required in only one state, and since medical services were limited, recording of the incidence of this disease was irregular.

The disease in question, although easily diagnosed on clinical grounds, was of unknown etiology at the time. Pathogenesis, mode of spread, immunity, and the role of social and environmental factors were matters of controversy. Many theories concerning the nature of the disease were proposed.

In an effort to determine the extent of the disease, a questionnaire was mailed to all practicing physicians in eight states in the Southeast regarding the number of cases they had seen in the preceding 5 years. Only onefourth of the physicians responded. They reported an increase from 622 to 7,017 cases over the 5year interval.

The following year, a U.S. Public Health Service epidemiologist was assigned to investigate the epidemic. After some preliminary observations, a field survey was conducted to determine the extent and the nature of the disease.

Question : Into what general etiologic categories might one divide diseases that can occur in epidemic form?