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 
isin 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- 
