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The seasonal variations in disease, as is the case in the spread of infective diseases generally, depend upon three primary factors: (1) the presence of the micro-organisms of adequate virulence and infectivity; (2) the means of transmission to (3) the susceptible tissues of the susceptible individual. All other factors, in this relation largely climatic, can act directly only through these three primary factors and are therefore termed secondary. The paper is largely an attempt to express the effect of the variations in the secondary factors in terms of the alterations they produce in the primary factors. The total effect of the three primary factors in the spread of infection is termed “dispersability” and a ratio or measure of dispersability is described. This ratio emphasizes occurrences in the pre-epidemic period.
The relationship of temperature and humidity to the alimentary, respiratory and percutaneous diseases and to the ectodermoses, is investigated. In the autumnal group of infections, attention is drawn to the action of carriers in causing a rise in dispersability in the spring and the frequent occurrence of a double wave of dispersability.
The effect of climatic changes is often cumulative, and this is most evident in the autumnal group of infections, the seasonal occurrence of which cannot be directly explained by temperature changes, or even by the action of light on phagocytes. The possibility of vitamin deficiency or excess producing cumulative effects is considered.
Secular changes in seasonal periodicities are investigated and these are associated with other changes, such as the intrinsic periodicities, mode of spread, etc., the combined changes being described by the terms aggradation and degradation of disease.
Slides covering the majority of the infective diseases of temperate climates and exhibiting the secular changes in Liverpool, and various geographical differences, in some 175 curves displayed.