70 llanual of Veterinary Microbiology. 



predisposed, enfeebled, hence less effectively prepared 

 for defense. 



IMMUNITY. 



Immunity consists in the absence of receptivity; 

 it may be natural or acquired. Acquired immunity is 

 most frequently consecutive to a first attack of the 

 disease. A large number of microbic affections do 

 not recur; they leave behind them organic changes, 

 inappreciable directly, but more or less permanent, 

 which oppose themselves to the later development of 

 their germs ; the individual, once recovered, is vacci- 

 nated. This is the case in variola, vaccinia, sheep 

 pox, syphilis, charbon, roiiget, fowl cholera, pleuro- 

 pneumonia, etc. Other diseases, on the contrary, can 

 reappear several times in the same individual ; such 

 are gonorrhoea, simple chancre, diphtheria, erysipelas, 

 tuberculosis, glanders, etc. In the case of these two 

 last diseases we see, in reality, specific lesions super- 

 vene at periods very remote from the time of the first 

 appearance of the affection, which would not happen 

 if one or more first attacks of the disease had vacci- 

 nated the organism. Immunity may also be com- 

 municated by artificial means, and the process by 

 which this is effected is known as vaccination. 



The duration of acquired immunity varies greatly 

 according to the disease and the individual concerned. 

 In general, immunity consecutive to the natural dis- 

 ease is more permanent than that conferred by vacci- 

 nation. 



Patient researches have thrown a little light upon 

 the nature of immunity, and shown that it operates, 

 at least in part, through the active intervention of 

 the organism which possesses it. 



