CHAPTER VI. 



THEORIES OF INFECTION, IMMUNITY, AND RECOVERY. 



THE tissues of the animal body under the normal 

 conditions of life are, as we have seen, unsuitable for 

 the growth of the great majority of the varieties of 

 bacteria. Indeed, only a very small number of the 

 parasitic bacteria find the conditions really satisfactory, 

 and even these must find a point of entrance into the 

 body. 



In seeking to account for the difficulty which to a 

 greater or less extent all bacteria find to growing in 

 the tissues of the living body, we cannot find it either 

 in the insufficient or excessive concentration of the 

 nutritive substances, nor in the temperature, nor in the 

 reaction; for although some of these conditions may be 

 unsuitable for some bacteria, they are all suitable for 

 many, and thus cannot constitute the fundamental ex- 

 planation of either natural or acquired immunity. A 

 possible ground, for the inability of the bacteria to 

 invade living tissues, might be thought to be found in 

 the fact that the nutritious material in the living cells 

 is in a form which the bacteria cannot readily assimi- 

 late; but, if this be true to a certain extent, it does not 

 adequately explain why the bacteria do not develop in 

 the nutritious fluids, so abundant about and in the body 

 tissues, nor does it account for acquired immunity. We 

 are thus driven to the conclusion that the. body fluids 

 themselves contain substances which are directly dele- 



