162 INFECTION AND IMMUNITY. 



VII. On first thought it would seem that immun- 



ity can have no place in an infection of loner dura- 

 Diseases. . / 



turn, from which recovery is rare or does not occur. 



This, however, is not necessarily true, and the very 

 chronicity of the infection may in some instances 

 depend on the establishment of a certain degree of 

 acquired immunity. It is, of course, possible that 

 in other instances chronicity depends on a low 

 degree of virulence on the part of the micro- 

 organisms or a low natural susceptibility on the 

 part of the host. Sleeping sickness may be taken 

 as an example of a chronic disease, the prolonged 

 course of which, in all probability, depends on the 

 formation of protective substances. This is indi- 

 cated from the fact that an acute is followed by a 

 chronic stage. At the height of the acute stage 

 trypanosomes are very numerous in the blood, but 

 after a time their number decreases and eventually 

 it is difficult to find them except in organs which 

 serve as reservoirs for them. Ehrlich speaks of 

 this type of immunity as Immunitas non sterili- 

 sans. It may disappear more or less completely, 

 its disappearance being marked by a recurrence of 

 acute trypanosomiasis. 



The existence of this temporary immunity to 

 trypanosomiasis was demonstrated in mice by 

 Franke. When mice, infected with mal de caderas 

 (a variety of trypanosomiasis) are given an injec- 

 tion of a sufficient quantity of "trypanrot," all the 

 trypanosomes are killed and the cure is immediate. 

 If a smaller quantity of "trypanrot" is injected, it 

 may still be sufficient to free the circulation from 

 parasites for twenty or thirty days, after which 

 general invasion again occurs. During this period 

 of comparative freedom from parasites the animals 



