l6 QUATERCENTENARY STUDIES IN PATHOLOGY 



Summing up these two curves, the conclusions which one must 

 necessarily draw, are that human plague commences immediately after 

 the increased incidence of rat plague, and that variations in the latter 

 occasion variations in the former. 



A close study of these charts is almost bound to lead one to think of 

 some relation between the two outbreaks. It may be said that both run 

 independently of each other, only the epidemic appears about a fortnight 

 later. However, it would be difficult to imagine such a regular system 

 in the incidence of an infectious disease, when compared with an 

 epizootic. We may imagine the epidemic of plague commencing 

 practically at the same season during each year, but that it should 

 regularly appear within a definite incubation period after the advent of 

 the epizootic is difficult to get away from, unless we admit of the 

 existence of a definite relationship. Again, it is found that the plague 

 epidemic does not commence at a definite period during each year. In 

 1902 it was not present until the middle of April. In 1903, it began 

 about the ist of February ; in 1904, as we shall see later, its commence- 

 ment was even later than during 1902. 



If we compare these dates with that of the advent of the epizootic, 

 the following is found : — In 1902 the epizootic began about the ist of 

 April ; in 1903 it began about the middle of January, and in 1904 it will 

 be found that a similar condition of aflfairs was present. Thus it is 

 evident that, apart from the season of the year, epidemic plague appears 

 to be preceded by epizootic plague, and that the interval of time 

 between the outbreaks varies from ten days to a fortnight. 



(c) During the year igo/j.. 



The condition of affairs during this year compares very favourably 

 with the points already alluded to under the two previous years. The 

 attached Chart V. itself explains the constant relation between the degree 

 of severity of the epizootic and the epidemic. During the first week in 

 January it is seen that rat plague was present to a considerable extent in 

 the Colony. The epizootic rose step by step, until the beginning of 

 February, when a rather erratic jump upwards took place. This sudden 

 increase in the amount of rat plague was followed by the appearance of 

 human plague after about ten days. 



(230) 



