For a long time epidemiology was a term associated with the study of outbreaks of disease which were sudden and large-scale. The attempt to find common causative agents to which the majority of cases could be attributed has provoked a literature worthy to rank with some of the best detective fiction. So many of the guilty agents have now been either liquidated or rendered impotent that infectious illnesses have ceased to occupy the centre of the public health stage, and have yielded place, as objects of concern, to such chronic diseases as cancer, rheumatism, heart disease and the schizophrenias. These diseases do not generally show explosive outbreaks, although mental disorders have been known to behave in this way, as witness the outbreaks of Dancing Mania which originated in Italy in the thirteenth century. All this has led to a more exact concept of epidemiology as “the study of the distribution of a disease or condition in a population and of the factors that influence this distribution” (Lilienfeld (1)).