PATHOGENIC BACTERIA. 267 



During epidemics of plague it has been noted that rats 

 may die in large numbers, and plague bacilli have often been 

 recovered from the bodies of such rats. The systematic 

 destruction by health departments of all the rats possible is 

 important where an epidemic is present or is feared. The 

 same applies to mice. The agency of fleas as carriers of 

 the bacilli has been suggested, but has not yet been proved ; 

 this is equally true as to flies. 



The greatest care must be used in working with the 

 bacillus of plague. A number of fatal results have occurred 

 through it in laboratory investigators. 



Haffkine has invented a method of protective inoculation 

 against plague by the injection of cultures of plague bacilli 

 which have been sterilized by heat, and a little carbolic acid 

 added. An active immunity, w-hich is quite lasting, it is 

 maintained, may be secured in some days. The injection 

 is sometimes followed by considerable constitutional dis- 

 turbance. This method seems likely to be of considerable 

 value. 



Yersin and others have prepared protective sera on the 

 same general principles used in making other sera for effect- 

 ing passive immunity. It is hoped they may be useful in 

 producing quickly a temporary immunity; and the outlook 

 for their employment in the treatment of the disease is 

 very encouraging. 



An agglutination reaction has been described; it is not 

 likely to be cf great value in diagnosis. 



The period of incubation in this disease is from two to 

 seven days. It has occasionally appeared in civilized 

 countries during recent times though not to a very seri- 

 ous extent. Among the localities of importance to us it has 

 recently visited the Philippine Islands, California and 

 Mexico. It has ravaged the southeastern part of Asia 

 within a few years. In the Middle Ages, and in succeeding 

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