272 



PATHOGENIC BACTERIA IN MILK 



" The incidence of the attacks by age," writes Dr Davies, " is in 

 this outbreak very much controlled by the special circumstances of 

 the case, and especially by the fact of the milk having been dis- 

 tributed and used amongst large school boarding-houses. As the 

 school age here extends up to 20 years, there is a notably 

 large number of attacks at ages 1 5 to 20. But at all school ages 

 from 5 upwards the attacks are significantly high." He further 

 adds, " The excess in attacks upon younger children is significant 

 in conjunction with the heavy incidence upon females and upon 

 domestic servants of the communication of the infection by milk." ^ 



In this same outbreak Dr Davies was able to show very clearly 

 the age incidence of attack in relation to the persons coming 

 directly in the way of infection by living in the houses supplied 

 with the implicated milk : — 



So much for external circumstances affecting age incidence. 

 There is also demanding consideration the specific disease in ques- 

 tion. Scarlet fever and diphtheria are, in the main, children's 

 diseases ; typhoid fever is, in the main, a disease of young adults. An 

 exception to either of these common standards would, of course, 

 raise a prima facie suspicion of milk infection. The above is an 

 excellent example. Another occurred at Barrowford in Lancashire 

 in 1876, when, out of fifty-seven persons attacked with milk-borne 

 typhoid, as many as twenty-five were under lo years of age. 



2. Incubation Period 



It is well known that, between the time of infection and the 

 time of invasion, or symptoms of disease, there is a longer or 

 shorter period of incubation. A man comes in the way of infection, 

 say, of smallpox or typhoid fever. But a period of about fourteen 

 days passes before the disease commences to show itself In scarlet 



^ Loc. cit, pp. 93 and 94. 



