106 APPLIED BACTERIOLOGY 



theory). Again, some feeble pathogenic organisms, as 

 in the case of attenuated tetanus bacilli, become greatly 

 exalted in virulence when cultivated with the Proteus 

 vulgaris. 



Immunity. — Immunity means the protection which is 

 afforded to the animal body against pathogenic organisms 

 under certain conditions, which immunity may be either 

 natural or artificially acquired. When an animal is non- 

 susceptible to a given disease, the immunity is said to be 

 natural, but when the protection is afforded by a previous 

 attack of the disease, it is said to be acquired immunity. 



In considering the action of disease germs on animals, 

 one cannot fail to be struck with the remarkable differences 

 which the same organism produces when injected into 

 different animals. 



The organism that invariably produces a fatal disease in 

 one animal may, when introduced into another animal, 

 either produce a mere local affection of no particular 

 moment, or possibly no effect whatever. 



For example, a virulent culture of the Bacillus tubercu- 

 losis, if inoculated into a guinea-pig, will produce general 

 tuberculosis, resulting in the death of the animal ; but in 

 man a local tubercular infection, as, for instance, a post- 

 mortem wound, usually produces only a slight local lesion, 

 which after a time heals up completely. Cases, however, 

 occur in which such lesions are followed, sometimes after 

 considerable intervals, by generalised or local tuberculosis. 



When an organism is capable of producing specific 

 disease in an animal, that animal is said to be ' susceptible ' 

 to that disease. 



The ' susceptibility ' varies greatly in degree, even for the 

 same kind of animal, the following being some of the most 

 important factors regulating the degree of susceptibility : 



1. The age of the animal, young animals being often 



