74 MILK AND THE PUBLIC HEALTH CHAP. 



and the varying degrees of susceptibility are considered, it is 

 obvious that only a portion, possibly a small proportion, of 

 those taking the incriminated supply will be specifically infected. 



A difficulty which has to be taken into account is the 

 very considerable interchange of milk which takes place in 

 large towns and elsewhere. This may make the task of com- 

 parison between the incidence of infection and the individual 

 milk supply a difficult one. Cases which appear to be sup- 

 plied with milk from quite a different source, and so to be 

 etiologically unconnected with the outbreak, may be in fact 

 connected, since the dairyman supplying them may have 

 reinforced his own deficient supply with a churn of the 

 implicated milk. Very careful and detailed inquiry is some- 

 times required to unravel these byways of infection. 



The cases due to secondary infection have to be carefully 

 traced and recorded separately in unravelling the epidemio- 

 logical features of any outbreak. 



2. The Outbreaks are explore in Character. Their ex- 

 plosive nature is a characteristic feature of both milk and 

 water outbreaks. Sometimes they are extremely explosive, 

 the bulk of the cases starting the disease within one or two 

 days of one another. The epidemiological picture will vary 

 somewhat according as to whether the specific infection is a 

 single one, for example at one milking, or is in operation either 

 continuously or intermittently for a number of days. 



In many milk-borne outbreaks there is but little secondary 

 infection, and the decline is usually very sudden on removal 

 of the source of infection. On the other hand, long-continued 

 infection with continuous notification of cases without abrupt 

 decline has been a feature of a good many milk outbreaks. 



3. The Incidence falls upon the Milk-consuming part of 

 the Community. Social, The poorer classes consume but 

 little milk compared with their richer neighbours. Milk- 

 spread outbreaks show a higher incidence upon the classes 

 who habitually consume more milk. 



A striking example of the relationship between social 

 position and attack incidence is furnished by Sir William 

 Power in his report upon the milk-borne scarlet fever at 

 Wimbledon 1 in 1887, as shown in the following table: 



1 Report of Medical Officer, Local Government Board, 1S86, p. 327. 



