SPIRILLUM CHOLERA ASIATICS. 465 



But even as regards the peculiar laws which seem to 

 govern the local and seasonal spread of the disease, 

 we may easily find, among the important factors 

 mentioned above, some which completely explain the 

 repeated or permanent insusceptibility of some places, 

 or the constant preference for a definite season of the 

 year. 



Thus the occurrence of cholera in Germany in late Explanation 



of the seasonal 



summer and harvest can be explained by the presence 01 predisposition 

 various predisposing factors during these months. It is m Geru 

 during this season, as Pettenkofer has pointed out, that 

 the level of the ground-water is lowest, and that conse- 

 quently the upper layers of the soil are driest ; the exist- 

 ence of a definite drying zone has, as was pointed out above, 

 a markedly favourable influence on the preservation and 

 increase of the sources of infection. The cholera months 

 are also those in which the largest numbers of insects 

 are present, and these undoubtedly take part in the 

 transport of the germs ; they are also the months in 

 which people eat the greatest quantity of raw food, which 

 is also well suited for the transport ; but above all in 

 those months there is a marked individual predisposition 

 on the part of a relatively large portion of the population 

 on account of the frequent gastric disturbances, the 

 causes of which have been previously referred to. It is 

 easy to understand that where this dry zone is absent 

 and with it a number of sources of infection, where the 

 various means of transport referred to are wanting, and 

 where there is no wide-spread individual predisposition, 

 the chances for the spread of cholera are very unfavour- 

 able, and that therefore in our climate it does not 

 usually spread at all during winter and spring, or at 

 most only a few individuals are affected. At times, 

 however, in spite of the unfavourable season of the 

 year, some of the other predisposing factors may be 

 so markedly developed that an exceptional epidemic 

 may occur even in the middle of winter or in spring. 



There are also a number of factors which explain the Explanation 

 extinction of an epidemic after its longer or shorter dura- tk>r> h of extm<; ~ 

 tion. In this respect one important point is, that soon epidemics. 



30 



