Relations between human diseases, particularly their incidence and seasonal changes, have long been studied by a number of scholars. It has recently been found that the variation of morbidity and mortality by disease is dependent upon socio-economic conditions as well as upon natural environments, meteorological and geographical, whereas the seasonal cycle has so far been reflected more or less in the distribution of various ailments. Take, for instance, dysentery, which formerly raged in the hot months. It now shows a very small hill in summer, and its morbidity in winter has been increasing year after year in proportion to the gradual decrease in summer.
The seasonal variation of mortality by disease has also undergone marked changes. And these changes can be classified into two types: transitory and reversing.
The transitory-type change is seen in cancer, which took the biggest toll in August in the first decades of the current century, but the mortality peak of which has moved from summer to autumn, October in particular. The coefficient of seasonal variation has shrunk from about 0.1 to 0.04, or nearly one half.
The reversing-type change may further be divided into two subtypes, A and B. In subtype A the summer peak of mortality has caved off into a valley, while on the other hand the winter level, because of its minor decline, has been closed up as a new summit. Falling under this category are gastritis-enteritis group avitaminosis (hen-ben) and tuberculosis. In the case of Subtype B, there were formerly two peaks, one in summer and the other in winter, but now the summit remains intact only in the cold season, whereas it has completely disappeared in the hot season. Diseases registering such seasonal variation of incidence are apoplexy, heart disease and senility.
What are the factors responsible for such seasonal changes in mortality ? As for the infirmities of age, high mortality in summer may be considered as having come from the bad influence of the hot season upon old patients and the lack of medical services for them in former times. The recent development of medicine and pharmacology, however, has come to enable old cases to survive the effects of summer heat, but they cannot endure winter cold. This also may be mentioned as one of the factors. As another cause may be mentioned the recent contraction of seasonal variation in major physical functions of the human body, such as basal metabolism and blood specific gravity of red corpuscles.
The retardation of mortality claims closer attention. After the Second World War, mortality markedly declined for many diseases, but it has come to mark time of late. Signs of such retardation can be seen in every season for pneumonia, enteritis, tuberculosis and other germ diseases. On the other hand, incidence has been on the steady increase, particularly in winter, for apoplexy, heart disease and other senile maladies, probably because old people have been increasing relatively in the population structure.
Diseases showing marked seasonal variation in morbidity and mortality have long been called “ seasonal diseases ”. But this is not an exact definition, gividg an impression that seasonal changes are reflected, as they stand, in the occurrence and incidence of diseases. In fact, many of the so-called seasonal diseases have proved not seasonal in nature.
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