H2 PATHOGENIC BACTERIA. 



resemblance to natural immunity and to attain about 

 the same degree of resisting power as is observed in 

 natural immunity. Thus, after a horse recovers from 

 traumatic tetanus it remains for a long time immune to 

 the germs of tetanus and resistant to their toxin. It is, 

 however, susceptible to intoxication with tetanus poison 

 in moderate doses. 



Experimentally acquired immunity may differ from 

 accidentally acquired immunity in that the resistance to 

 both infection and intoxication may be increased to an 

 amazing degree, the animal being gradually accustomed 

 to the infection or intoxication until it can endure hun- 

 dreds of times the fatal dose for a normal animal. 



This process of habituation is technically called im- 

 munization or forced immunity, and is accompanied by 

 certain phenomenal qualities of the blood serum, which 

 becomes antimicrobic or antitoxic, or both. 



The greater number of the acquired immunities are 

 active and cytogenic. Passive immunity makes its ap- 

 pearance, however, among the acquired immunities only. 



That acquired immunity is not hereditary is amply 

 illustrated by the fact that though nearly everyone has 

 the diseases of childhood and becomes immune to them, 

 children are still born susceptible to measles, mumps, 

 chicken-pox, etc. 



I. Accidentally acquired immunity is the result of 

 accidental conditions that arise in nature. 



a. Infectiojt is the most frequent cause of this form of 

 immunity. Accidental infection commonly results in a 

 more or less permanent immunity to disease. Thus, 

 children born susceptible to measles, scarlatina, mumps, 

 and the other infectious diseases of childhood are 

 usually accidentally infected during early life, the sur- 

 vivors usually remaining immune thereafter. Occasion- 

 ally the immunity thus attained gives out and a rein- 

 fection becomes possible. Such cases are, however, 

 exceptions, not the rule. The immunity attained in 

 this way is usually so active that the organism is not 



