52 SPECIFICITY OF RICKETTSIA PROWAZEKI 



was taken daily by a uniform technic in order to give an indica- 

 tion of the probable results of the inoculations. At the time 

 these experiments were begun we were seriously handicapped 

 for lack of a sufficient number of normal guinea-pigs. Our 

 stocks of monkeys and guinea-pigs were suffering from para- 

 typhoid infections which in the guinea-pigs in the chronic stage 

 took the form of a pseudo-tuberculosis. We were forced to use 

 many guinea-pigs which had passed through this infection but 

 which had apparently recovered as evidenced by a period of a 

 week or more of normal temperature. In several instances this 

 infection was relighted as an effect of the typhus infection. It 

 apparently shortened the incubation period of typhus and pro- 

 longed the febrile period. The final conclusions as to the re- 

 sults were based upon one or more of the following controls: 

 histological examination of the brain and other organs for 

 lesions characteristic of typhus; subinoculation of other 

 guinea-pigs; reinoculation with blood of typhus patients as 

 a test of immunity. The human bloods used in testing the 

 immunity of the guinea-pigs at the conclusions of the experi- 

 ments were proved to be infective for fresh normal guinea-pigs. 

 Lice from Boxes xvn, xvm, xix, xxi, xxn, xxvm, xxxiv, 

 xxxvi, xxxvn, and xxxvm were used for these experiments 

 (see Table VIII of the 52 louse feeding experiments). Owing to 

 the importance of these experiments they are given in detail. 

 A summary of the results is presented in Table IX, p. 110. 



