66 INFECTION AND IMMUNITY 



bacilli ; while a larger number introduced at one time 

 may overwhelm the resisting power of the individual. 



The essential difference between a susceptible and 

 immune animal depends upon the fact that in one 

 the pathogenic germ, when introduced by accident or 

 experimental inoculation, multiplies and invades the 

 tissues or the blood, where, by reason of its nutritive 

 requirements and toxic products, it produces changes 

 in the tissues and fluids of the body which constitute 

 disease and may result in death. On the other hand, 

 in an immune animal multiplication of the germ and 

 consequent disturbance of vital functions does not 

 occur, or is restricted to a local invasion of limited 

 extent, in which the parasitic invader soon succumbs 

 to the resources of nature. This essential difference 

 evidently depends upon conditions favourable or un- 

 favourable to the development of the germ ; or upon 

 its destruction by some active agent present in the 

 tissues or fluids of the body of the immune individ- 

 ual ; or upon a neutralisation of its toxic products 

 by some substance in the body of the animal which 

 resists infection. 



Among the unfavourable conditions which may 

 be supposed to prevent the development of disease 

 germs in animals which have a natural immunity 

 against infection by them, we may mention, first, the 

 temperature of the animal. It is well known that 



