The monthly incidence of scarlet fever was studied by time-series analysis. Two types of periodic fluctuations were detected involving a bimodal short seasonal cycle and a long-term cycle whose period takes 5 years aproximately. The interactions of the respiratory transmition mechanism, person to person contacts and environmental temperature would explain the seasonal cycle bimodality. The long-term cycle would be explained by asinchronic fluctuations of the fraction of population assigned to "cases" and "susceptibles"...
