THE RELATION OF DIET TO DISEASE 421 
There are two important factors to consider in dis- 
cussing the influence of diet on intestinal bacteria: 
(1) The substitution of types, which frequently follows a 
monotonous diet, and (2) the change in metabolism of 
existing types of bacteria when dietary conditions are 
such that the intestinal medium at one or another level 
fluctuates in its content of usable carbohydrate and other 
nutrient. The nature and extent of these modifications 
and their effects upon the host vary greatly, not only 
qualitatively but quantitatively. An invasion of the tract 
by exogenous bacteria, as the dysentery bacillus, cholera, 
typhoid, etc., in food or water may lead to a more or less 
pronounced replacement of some of the normal intestinal 
types by these alien organisms, and to the production 
of disease. 
The importance of all the foregoing facts concerning 
the changes in the food, in the intestinal cultural substrata 
and in the advent of new kinds of organisms was emphati- 
cally demonstrated in the marked fall in gastrointestinal 
diseases in carnivores after proper screening of meats. 
The simple protection of the food given to these animals 
eliminated the air bacteria which, entering from dust and 
flies, alter the chemistry of the meat before consumption 
or change the flora of the intestine after consumption. 
Normal organisms, or types indistinguishable from them, 
may multiply, through unusual conditions, extend their 
normal habitat, and eventually lead to abnormal reactions 
detrimental to the host. These facts throw considerable 
light on the site and character of gastrointestinal lesions 
found in various orders, a subject to be discussed more 
fully later. 
There are many intestinal disturbances of unknown 
causation, in some of which bacteria presumably play a 
secondary part. The primary disturbance is due to the 
products resulting from the action of bacteria upon food. 
Many toxic bodies are produced either before or after 
ingestion by the bacterial decomposition of carbohydrate, 
