CHAPTER V. 



IMMUNITY. 



THE condition of being insusceptible to an infective disease may be 

 either natural or acquired. In studying the pathogenic organisms 

 several examples of natural immunity will be encountered. The 

 bacillus of septicaemia, so fatal to house mice, has been shown to 

 have no effect upon field mice. The bacillus of anthrax is innocuous 

 to cats and white rats. The bacterium of rabbit septicaemia is 

 equally inert in dogs, rats, and guinea-pigs. The immunity may 

 be as in these cases complete, or only partial. . Ordinary sheep are 

 very easily affected with anthrax, but Algerian sheep succumb only 

 to large doses of the virus. Natural immunity may not only be 

 characteristic of certain species, but it may occur in certain indi- 

 viduals of a susceptible species. The same immunity occurs in man, 

 for certain individuals, though equally exposed during an epidemic 

 of small-pox, may escape, whereas others readily fall victims to the 

 disease. 



Acquired immunity is illustrated by the protection afforded by 

 one attack of the exanthemata against subsequent attacks. Thus 

 one attack of measles or small-pox, as a rule, affords complete 

 protection. A knowledge of the immunity resulting in the latter 

 case led to the introduction of inoculation of small- pox as a 

 protection against natural small-pox. 



Immunity may be acquired by acclimatization, for the inhabit- 

 ants of tropical climates are less susceptible to the diseases of the 

 country, malarial fevers, for instance, than strangers. 



In civilised communities also, there appears to be a degree of 

 acquired immunity, for infectious diseases like measles introduced 

 among savages or isolated communities have assumed the most 

 malignant type. 



The immunity acquired by protective inoculation constitutes, in 

 connection with the study of pathogenic micro-organisms, a subject 

 of pre-eminent interest and importance. Pasteur, in his researches 



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