CHAPTER X. 



IMMUNITY. 



THE condition of being insusceptible to an 

 infective disease may be either natural or acquired. 

 In the description of the pathogenic organisms 

 several examples of natural immunity will be en- 

 countered. 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 pigs, cats, white rats, and to adult 

 dogs, asses, and horses. 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 only succumb to large doses of the virus. 

 Natural immunity may not only be characteristic 

 of certain species, but it may occur in certain 

 individuals of a susceptible species. The same 

 occurs in man, for certain individuals, though 

 equally exposed during an epidemic of small-pox, 

 may escape where others readily fall victims to 

 the disease. 



