ABSTRACT
All epidemic diseases are to be accounted for on the principle of natural causes. Sydenham was perhaps the first to state clearly the distinction between conceptions which have often been confused. There was first the varying effect of a given maleficent influence according to the condition of the person affected by it. There is secondly the variation of disease considered to be due to external conditions which determine in an invaded person one or another disease, apart from any specific difference in the infecting agent. Thirdly, there is the phenomenon of cyclical prevalence of a disease, which constitutes the chief value of the idea of epidemic constitution. Cyclical influences are manifested in a number of infectious diseases. The history of plague, cholera and typhoid, and the demonstration which preventive medicine and public health have made that these diseases can be reduced to insignificance, should render us not entirely without hope that eventually the same may be said regarding influenza.
