INTRODUCTION Although the term, epidemiology, has in the past been applied generally, if not exclusively, to the study of communicable diseases, so far as concerns their development among groups, or their manner of transmission from the infected or the carrier to the susceptible person, the methods of such investigations can often be employed with advantage in analyzing the causes and distribution of other diseases. The essential and distinctive feature of epidemiology is the study or knowledge of that part of the natural history of a disease which determines its occurrence or distribution quantitatively and qualitatively in groups of persons, particularly as one case of sickness is related to others, by some common factor of age, sex, race, color, occupation, geographical distribution, or determining element in heredity, environment or hygiene, distinct from the study of disease as it affects the life history of the person by modifying the structures and functions