602 
ANNALES DE L’INSTITUT PASTEUR 
Conditions influencing the résistance of the possible host ; 
Frequeney and completeness of recoveries from infection 
(occult or manifest). 
Of the factors which détermine the occurrence of infection 
and the course taken by the disease, there are two which are of 
spécial importance, viz., the résistance of the possible host to 
the infecting parasite, and the pathogenic power of the parasite 
itself. In the case of the tubercle bacillus, it lias been 
demonstrated clearly, as lias been previously noticed, that 
tubercle bacilli obtained from various kinds of animais are 
not usually equally pathogenic to certain experimental animais. 
As to the résistance of the host, apart from the different 
degrees of résistance which each kind of animal offers to 
bacilli coming from various other kinds of animais, there are 
various conditions which influence the résistance of individuals 
of the same species to any one type of bacillus. 
The way in which the parasite and the host reacl upon each 
other can be studied most successfully when the infective 
power of the bacillus and the résistance of the host are almost 
equally balanced, as lias been done by Metchnikoff in his 
classical study on experimental tuberculosis in the Meriones. 
His description of the struggle belween the tubercle bacillus 
and the giant cell of the Meriones is very suggestive of what 
must liappen if, for some reason or other, the physiological 
activity ofeither the cell or the bacillus is reduced or enhanced. 
The sécrétion by the bacillus of concentric layers of a protec- 
ting material, and the précipitation of calcareous sait in the 
parts of the giant cell which are, in ail probability, affected by 
the bacillary toxines, indicate at least part of the mechanism 
of attack and defence. 
In the course of experiments made with the view of ascer- 
taining the effect which the action of various agents had upon 
the virulence (1) of tuberculous products of human and bovine 
sources, one is sometimes able to observe evidence of a réduc¬ 
tion in the pathogenic power of bacilli, and tliis réduction may 
be sufficient to render the cobaye, capable of resisting bacillary 
(1) The word virulence is here used in its most general sense, including 
ail that renders the organism capable of producing irritative and toxic lésions 
and of multiplying and spreading in its host. 
