70 MILK [vi 



than fourteen epidemics of the former, and seven of 

 the latter, to this source in England ; and while we 

 have not the same evidence with regard to other 

 infectious diseases, there can be little doubt that 

 many of them in the past have been spread in this 

 manner. There would seem to be a certain class of 

 bacteria that are quite harmless when taken into the 

 system, under ordinary conditions ; if, however, the 

 vitality of the body is in an impaired state they 

 become a source of disease. Such bacteria are prob- 

 ably common in milk, and are doubtless the cause 

 of gastric and intestinal disturbances so common in 

 young infants during the summer months (summer 

 diarrhoea and cholera infantum). They are able to 

 produce certain by-products which have a poisonous 

 effect when taken into the susceptible digestive tract 

 of the infant. 1 The importance, therefore, of a know- 

 ledge of the bacteriology of milk, in the interests of 

 public health, will thus be seen to be enormous. 6 



Although the existence of the world of micro- 

 organisms was drawn attention to as far back as 

 1675 by Leeuwenhoek, who discovered them first in 

 the saliva of the mouth, still it is only within the 

 last thirty or forty years that any strides in the 

 science of bacteriology have been effected. Still 

 more recent is our knowledge of their enormous 

 distribution in air and water, and our conception 



1 See Bulletin 44, Ag. Exper. Stat., University of Wisconsin. 



