CAUSES OF DISEASE. 225 
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 areignorant. 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 
