454 SPIRILLUM CHOLERA ASIATICS. 



pression that the epidemics are dependent on some 

 factors which vary according to the season on a seasonal 

 predisposition. 

 Extinction of A third striking fact with regard to the mode of spread 



the epidemics. /.-IT , -, , -, , , -i ^ 



of cholera is that in one place the epidemic is often 

 extinguished, while in other and neighbouring situations 

 it continues, and that this extinction is observed both 

 after a short and moderate amount of disease, and after 

 a long continued and violent outbreak of the plague. 



The question arises whether these puzzling facts as 

 regards the epidemic spread of the disease, which have 

 excited the greatest interest during the last few years, 

 may not possibly be solved by a more accurate analysis 

 of the mode of infection on the same lines as that above 

 given in explanation of the transmission of cholera from 

 individual to individual. 



What are the It is a priori probable that the peculiar local and 

 factor? which seasonal distribution of cholera epidemics is brought 



y the action of several factors varying according 

 of cholera to local and seasonal conditions, and depending partly 

 on the sources of infection available for the spread of 

 the epidemic, partly on the paths of transport from these 

 sources to the exposed individual, and partly on the 

 susceptibility of the latter. In studying the infection of 

 an individual we become acquainted with a number of 

 influences by which the sources of infection could be 

 multiplied or reduced in number, the paths of distribu- 

 tion enlarged or narrowed, and the individual suscep- 

 tibility increased or diminished. These factors are also 

 undoubtedly of equal importance in the epidemic dis- 

 tribution of the disease ; for we must remember that 

 an epidemic only originates and spreads when a chain 

 of new cases follows the first in a continuous series, 

 and that it disappears when this chain is broken. 

 Just as the inoculation of the individual requires 

 certain favourable chances, just as all those infections 

 which serve for the continuance of the chains are 

 influenced by chances of all kinds, so the epidemic 

 will come to an end, chiefly because, owing to external 

 circumstances, the sources of infection become less 



