96 INFECTION AND IMMUNITY. 



toxins which they generate, or on account of the highly poi- 

 sonous property of these toxins. Certain bacteria and their 

 toxins have been proved to exercise a selective " affinity " for 

 certain tissues ; e. </., the B. tetanus and B. botulinus have a 

 selective affinity for the nervous system. 



1. The quantity of bacteria used plays an important part 

 because there is a more or less marked natural resistance in 

 the animal body to the action of bacteria or their poisons. 

 When these are introduced in small quantity only, they fail 

 to produce any effect, and it requires a certain definite amount 

 of bacteria to produce disease in the animal body. This 

 amount varies with the species of the bacteria. 



2. The condition of the bacterial culture when introduced 

 into the animal body is an important factor in the subsequent 

 course of the infection, for bacteria under different conditions 

 secrete toxins which are more or less injurious, and the same 

 bacteria grown under the same conditions are able at different 

 times to produce toxins of more or less virulence. When 

 the condition of growth or the environment of the bacteria 

 varies, their cultural aspects and the amount of toxins they 

 are able to produce vary also. So much so is this the case 

 that bacteria are grown under peculiarly disadvantageous 

 surroundings high temperature, or the addition of a small 

 proportion of antiseptics to their cultural fluid so as to pro- 

 duce bacteria of less virulence in other words, to attenuate 

 them. 



Methods of Attenuation. Bacteria from young liquid cult- 

 ures are known to be more virulent than those from older 

 cultures. Again, cultures are made through the body of 

 resisting animals so as to diminish the virulence of the cult- 

 ures. Or, again, the cultures are passed through artificial 

 media for a number of generations to diminish their virulence. 

 The converse of this happens also, and bacteria grown or 

 passed through the bodies of susceptible animals acquire more 

 and more virulence. 



3. The method of introduction of the bacteria contributes 

 considerably to the degree of infection from the fact that 

 nearly all bacteria have certain affinities for different tissues 

 of the body where they exert their most baneful influence, 



