378 INVESTIGATION, ETC., OF MILK-BORNE EPIDEMICS 



illness on the first two days of the nine days over which the 

 notifications were received. Here then was an outbreak not 

 occurring during nine days, but during two days. Consequently 

 the source of the epidemic had to be sought previously to those 

 two days, and approximately the length of the incubation period 

 before the two days. Contamination of the milk supply was 

 detected. But if the cases had been traced by the date of their 

 notification and not the onset of the symptoms, it is probable 

 that the infected milk would not have been discovered, or at all 

 events not so rapidly. 



The incubation period and approximate infectivity of diphtheria, 

 scarlet fever, and typhoid fever, are (according to Goodall and 

 Washbourn) as follow : — 



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Note.— By Quarantine Period is meant the length of time during which a susceptible person, 

 who has been exposed to infection, is to be isolated. 



The importance of a careful inquiry into each individual case 

 in an investigation into milk-borne outbreaks cannot well be 

 exaggerated. Not only do the facts elicited frequently reveal the 

 exact channel of infection, but ways and means of preventing 

 further spread are thus discerned. 



In the investigation of suspected milk-borne outbreaks of disease 

 the third step is certainly of the nature of an inquiry into the 

 relationship of the milk supply : in short as to whether or not there 

 is a community of the milk supply. Suspicions of such community 

 may first be aroused by a detection of some of the characteristics 

 of milk outbreaks {see p. 262). There are various ways in which 

 the matter may be followed up. The essential fact, in this connec- 

 tion, as to each patient is recorded on the investigation form discussed 

 above. The relationship of a particular dairy or milk-round and the 

 occurrence of cases of the disease will be obtained by a considera- 



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