CONTAMINATION WITH ORGANISMS 105 



be spread by means of milk. In some cases, no doubt, 

 epidemics have wrongly been ascribed to the use of milk, 

 for their origin might be explained in another way, but 

 a very great number of cases remain wherein the respon- 

 sibility of milk as a distributor of contagion cannot be 

 doubted. 



[In most cases, the conclusion that a given outbreak 

 of disease is caused by the use of infected milk does not 

 rest on the evidence of the discovery of the specific 

 organism in the milk, but on the manner in which the 

 outbreak occurs and upon the various attendant circum- 

 stances. For this there are two reasons ; first, none of 

 the specific germs may be present in the drop or two sub- 

 jected to examination, even though they are relatively 

 numerous in the whole supply, for such organisms are 

 not in solution and are not evenly distributed and, sec- 

 ond, infectious diseases do not occur until after a certain 

 period of incubation, and when this time has elapsed the 

 milk that carried the specific organisms is no longer in 

 existence. But the evidence of the transfer of such dis- 

 eases by milk is none the less convincing. It is as clear, 

 for example, as the evidence that typhoid fever is car- 

 ried by contaminated water, and this evidence is re- 

 garded as sufficient to justify the expenditure by cities 

 of vast sums to secure pure water. 



Milk epidemics are often characterized, according to 

 the observations of Swithinbank and Newman 21 by the 

 following features : 



1. Special incidences of disease, as among the cus- 

 tomers of a certain milkman ; in families using the great- 

 est quantities of milk; among persons who drink most 

 milk and, for this reason, among women and children. 



2. A relatively short period of incubation. 



21 The Bacteriology of Milk, London, 1903. 



