124 MILK BACTERIA AND HEALTH. 



scarlet fever, or diphtheria, in a community the first thing 

 to do is to determine the locality of all- of the cases. Inas- 

 much as water is a means of distributing typhoid the next 

 task is to exclude, if possible, the common water supply. If 

 it is found that the epidemic is universally distributed 

 through the whole city the common water supply may be 

 thrown under suspicion' v but if the epidemic is confined to 

 certain parts of the city, or to certain streets, the common 

 water supply must be excluded. The next problem is to 

 determine the source of the milk in all of the families in 

 which there is a case of the disease. If it is found that all, 

 or nearly all, of the families attacked take milk from the 

 same milkman the milk is, of course, thrown under very 

 grave suspicion at once. In determining the possible source 

 of a milk epidemic it must always be borne in mind that, in 

 ordinary communities, the milk of each source is liable to be 

 distributed by more than one milkman. Milkmen freely 

 interchange milk with each other, either upon certain days or 

 regularly, and it is always necessary, in pursuing such an 

 investigation, to inquire whether the milk from the source of 

 suspicion has not been distributed by two or more milkmen. 

 It is frequently the case, therefore, that the milk from two 

 or three milkmen is placed under suspicion in this way, 

 although only one source itself is at fault. 



A method of tabulating the results and obtaining a 

 graphic illustration of the relation of the epidemic to the 

 milk supply is shown in the accompanying figure, which rep- 

 resents an actual instance of a scarlet fever epidemic due to 

 a milk supply (Fig. 23). It was found, in the community 

 in question, that there were 6,922 households taking milk 

 from 85 milkmen, and in all these households there were 

 only nine cases of scarlet fever. It was found, however, that 

 269 households (represented by circles in the figure) took 

 milk from three milkmen, and in these households there were 

 65 cases of the disease (represented by the black dots). But 



