PERIODIC IMMUNITY. 163 



are relatively immune,, which is shown by inability 

 to reinfect them with trypanosomes of the same 

 species. After this period is passed they succumb 

 very quickly. That the heightened resistance is 

 not due to the presence of a residuum of "trypan- 

 rot" in the body is shown by the fact that suscepti- 

 bility for other species of trypanosomes (as 

 nagana) is retained. In other words, this tem- 

 porary immunity is somewhat, if not absolutely, 

 specific. It probably is brought about by rapid 

 active immunization consequent on the disintegra- 

 tion of many parasites following the administra- 

 tion of the "trypanrot." 



The conditions in syphilis and piroplasmosis 

 would seem to be similar to that in sleeping sick- 

 ness; i. e., during and following general invasion, 

 reinfection from without does not occur, although 

 the disease is still active in some part of the body. 

 In both syphilis and trypanosomiasis reinvasion 

 from within may occur, presumably following the 

 disappearance of the temporary immunity. 



In still another infection, relapsing fever, it Periodic 

 would seem to be similar to that in sleeping sick- 

 and reinvasion alternate before the course is com- 

 pleted. It is not clear why the micro-organisms 

 are not entirely killed of? during the periods of 

 temporary immunity. Several factors may be 

 involved. During the periods of remission the 

 spirilla leave the general circulation and are found 

 in some of the solid organs, particularly the spleen. 

 At this time they may be protected to some degree 

 by an existence in organs which are relatively free 

 from germicidal agents. During this period also 

 the less resistant organisms may be destroyed and 

 those which remain may undergo an adaptation to 



