VACCINATION. 369 



a first, whilst in the human being and in the horse, anthrax 

 may occur again and again, though in all probability a 

 temporary immunity is conferred by an attack even in 

 these extreme cases. 



Cholera and typhoid fever both belong to the group of 

 diseases of which one attack usually confer a certain degree of 

 immunity against a second, the period during which this may 

 last, however, varying in different cases ; in some it extends 

 over a few years only, whilst in others it appears to be almost 

 permanent. The acute exanthemata, of which we may take 

 small-pox as an example, belongs to this latter class. 



Amongst the Turks the ingrafting or inoculating of small- 

 pox matter was very early resorted to as a routine practice in 

 order to produce a milder but protective attack of the disease. 

 We have a record of this in one of Lady Mary Wortley Mon- 

 tague's letters, dated 23rd March, 1718, and written from Bel- 

 grade, in which she describes the inoculation of her little 

 boy three years old. She says : " The boy was ingrafted last 

 Tuesday, and isat this time singing and playing, very impatient 

 for his supper ; I pray God that my next may give as good an 

 account of him. I cannot ingraft the girl ; her nurse has 

 not had the small-pox." The after progress of the patient 

 in this case was satisfactory, and, as is well known, Lady 

 Mary was afterwards instrumental in drawing attention to the 

 matter in this country. 



Then came Jenner's remarkable discovery that similar 

 immunity against small-pox might be conferred by the 

 inoculation of lymph from animals that were suffering from 

 what was known as cow-pox ; an attack of the one disease 

 in this case conferring an immunity against an attack of the 

 other. 



There has for long been the utmost anxiety on the 

 part of physicians and others to obtain some explanation 

 of these remarkable facts. Klebs and Pasteur explained 

 them on the assumption that during the course of a first 

 attack of a disease some material that was essential for the 

 nutrition of the pathogenic organisms that had by this time 

 been found to be associated with some of these diseases, 

 had been used up, and the supply being cut off, the organ- 

 isms were no longer able to exist in the body and exhibit 

 the characteristic evidence of their presence. This substance 

 must have been present in exceedingly small amount, as no 



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