SUDDEN ONSET OF MILK EPIDEMICS 275 



received unmixed milk from the infected dairy. Two other milk 

 vendors received a portion of their supply from the infected dairy. 

 This portion was mixed, as is the custom in general vogue, with 

 good milk derived from sources beyond suspicion. The houses 

 served by these two vendors with mixed milk were attacked at the 

 rate of 41 and 38 per cent, respectively as against 54 per cent for 

 the unmixed milk. Now it is not unreasonable to suppose that we 

 should find the length of the incubation period to depend in some 

 measure upon the degree of poisoning received. 



We venture to think that it is along these two lines, namely, 

 the development of the organisms in the milk and the degree of 

 poisoning, that inquiry' should proceed in future epidemics 

 attributable to milk, and in which there is evidence that the 

 incubation period is abnormally short. 



3. Sudden Onset and Rapid Decline 



Infectious diseases spreading by contact do not, commonly, 

 appear suddenly. In milk-borne and water-borne disease the 

 onset is sudden. Particularly is this so in milk infection. The 

 evident reason for this is that the cause commences to operate 

 suddenly. A cursory glance at the abstracts of various outbreaks 

 printed in subsequent chapters will illustrate again and again that 

 the majority of the cases occurred within a few days of each other. 

 Then, after allowing for the incubation period to elapse, it will be 

 found that the epidemic as suddenly ceases, immediately the supply 

 of poisoned milk is withdrawn or the milk itself boiled. These two 

 very simple features are almost pathognomic of milk-borne disease. 

 An exception to the rule occurs in those milk-carried epidemics 

 which are due, or are said to be due, to disease in the cow. In 

 these cases one may get an outbreak lasting for weeks, and 

 apparently attributable to no human source. Such an example 

 was the epidemic in North London in 1883-84, traced to the milk 

 supply of a farm at Hendon, and to be referred to later on. In 

 outbreaks originating in this way neither the onset nor the cessa- 

 tion is necessarily sudden, owing to the course of the disease. 



Under this heading must be mentioned another very important 

 and closely allied characteristic of milk infection. On making 

 investigation into the circumstances of such an outbreak it will be 

 found that multiple cases of the disease occur in the same house- 

 hold simultaneously and at the commencement of the outbreak 

 rather than later on. An epidemic spread by contact not only 



