THEIR MODE OF ACTION. 401 



1. Upon the presence of that substance from which 

 it was originally formed ; 



2. Upon the presence of a compound w^hich is 

 capable of being decomposed by contact with the 

 exciting body. 



If we express in the same terms the reproduction 

 of contagious matter in contagious diseases, since it 

 is quite certain that they must have their origin in 

 the blood, we must admit that the blood of a healthy 

 individual contains substances, by the decomposition 

 of which the exciting body or contagion can be pro- 

 duced. It must further be admitted, when contagion 

 results, that the blood contains a second constituent 

 capable of being decomposed by the exciting body. 

 It is only in consequence of the conversion of the 

 second constituent, that the original exciting body 

 can be reproduced. 



A susceptibility of contagion indicates the pres- 

 ence of a certain quantity of this second body in the 

 blood of a healthy individual. The susceptibility 

 for the disease and its intensity must augment ac- 

 cording to the quantity of that body present in the 

 blood; and in proportion to its diminution or dis- 

 appearance, the course of the disease will change. 



When a quantity, however small, of contagious 

 matter, that is, of the exciting body, is introduced 

 into the blood of a healthy individual, it will be 

 again generated in the blood, just as yeast is repro- 

 duced from wort. Its condition of transformation 

 will be communicated to a constituent of the blood; 

 and in consequence of the transformation suffered by 

 this substance, a body identical with or similar to 

 the exciting or contagious matter will be produced 

 from another constituent substance of the blood. 

 The quantity of the exciting body newly produced 

 must constantly augment, if its further transforma- 

 tion or decomposition proceeds more slowly than 

 that of the compound in the blood, the decompo- 

 sition of which it effects. 



If the transformation of the yeast generated in 



