149 



The rats examined for September, 1907, were very largely collected 

 from the badly infected districts; the remaining months give a truer 

 picture of the extent of the epizootic in the entire rat population. 



THEORIES AS TO THE CAUSE OF SEASONAL PREVALENCE. 



The marked seasonal prevalence of plague in man in San Francisco 

 may be given as additional proof of the association of the rat with 

 its spread. In the cold, rainy season, from December to April, the 

 epidemic ceases while the epizootic is apparently not influenced. 

 The anomaly is accounted for when we remember that the rat and 

 its parasites are very susceptible to cold and rain. It is then that 

 the animal seeks a warm, comfortable place from which it does not 

 venture until driven thence by dire necessity. In other words, the asso- 

 ciation of the rat with man is not so intimate in winter, while the re verse 

 is true of the relation of rat with rat. The rains, while interrupting 

 the overground migrations and domiciliary visits of rats, drive them to 

 overcrowded burrows and harboring places. Another factor should 

 be mentioned in this connection. Human fleas (P. irritans), and 

 probably rat fleas also, are markedly reduced in numbers at that 

 season of the year. We must conclude, therefore, that the seasonal 

 prevalence of plague in man is due to the effect of climatic conditions 

 upon the habits of rats and the life history of the insect carriers of 

 the bacilli. 



An examination of the foregoing should convince everyone that 

 all former theories as to the prolonged viability of B. pestis in 

 contaminated soil or in polluted streams, and of the periodical 

 spread of the infection therefrom, are no longer tenable. It may 

 also be stated that insanitary conditions, except in so far as they 

 furnish food and shelter to rats and other vermin, play no important 

 1342910 11 



