IMMUNITY AND SUSCEPTIBILITY. 1 13 



only enabled to resist bacteria with virulence equal to 

 those which caused the original infection, but also those 

 of much greater virulence. 



b. Modified infection, by which infection with a mod- 

 ified form of a disease, or with some closely related disease, 

 may cause immunity. This is best exemplified in the 

 vaccination against smallpox. The exact nature of vac- 

 cinia and its true relation to variola are not yet settled, 

 although the modern view, based upon a great amount 

 of evidence, is that they are the same disease, variola 

 a virulent form, vaccinia a modified, attenuated form. 

 The observations leading to the early experiments 

 and conclusions of Jenner were that milkmaids acci- 

 dentally contracting vaccinia from the cow did not 

 subsequently contract variola, having acquired immu- 

 nity to the one affection by having suffered from the 

 other. 



c. Aberrant diet may be a cause of immunity. The 

 experiments of Hankin upon rats have already been 

 quoted, and it will be remembered that when rats which 

 are refractory to anthrax are fed upon a strictly vegetable 

 diet their susceptibility is increased, while if they are fed 

 upon meat their immunity is increased. 



In the natural condition it is not probable, though 

 always possible, that an animal might select some un- 

 usual food the ingestion of which would be followed by 

 immunity to poisons. Ehrlich 1 found that when mice 

 were fed with food containing minute quantities of ricin 

 they developed immunity to ricin. It may be possible 

 that the immunity possessed by certain birds and mam- 

 mals against serpent's venom depends upon the fact that 

 they prey upon the snakes, and from ingested venom, 

 liver, or blood of the reptile acquire the resisting power. 

 Some authors assert that the snake-charmers of India, 

 who seem to be immune to cobra poison, become so in 

 consequence of making a habit of consuming some of the 

 venom every day. As this is scarcely compatible with 



1 Deutsche med. Wochenschrift, 1891, Nos. 32 and 44. 



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