CA USES OF DISEASE. 2 2 5 



affected, and pathologists are ignorant of the actual con- 

 ditions of wounds favourable to the development of the 

 micro-organism characteristic of this disease, and it is 

 equally certain that three or more individuals with open 

 wounds may be equally exposed to the virus and yet 

 some of them escape. This immunity may depend on 

 chemical, or fermentative changes, going on in the wound, 

 which produce a medium favourable to the growth and 

 development of the erysipelas-germ. On the other hand 

 it is possible that atmospheric and thermal conditions 

 may favour their development. 



The more these questions are studied the more we 

 perceive that the outbreak of infectious diseases depends 

 not so much upon the presence of micro-organisms 

 for, like the torula, they seem to exist everywhere 

 as upon the existence of suitable conditions, and as 

 yeast cannot grow and multiply without sugar, neither 

 can the poison of erysipelas, typhus, relapsing fever, 

 and the like, propagate without the presence of some sub- 

 stance produced in living bodies, of the nature of which 

 we are ignorant. This is well shown in Pasteur's researches 

 on fowl cholera : in this instance the microbe would not 

 live in the ordinary cultivation-media employed by 

 him, but when introduced into chicken-broth grew 

 rapidly. On a similar principle relapsing fever is 

 unknown except in times of famine, when the body- 

 chemistry is deranged by want of food, privation, and 

 hardships of every kind. 



There is yet another remarkable process which is a 

 modification of inflammation, viz., the repair of wounds. 

 When a wound is made in the tissues of an animal, and 



16 



