AGE AND SEX INCIDENCE 



269 



facts may be taken as follows : In scarlet fever 49 per cent of the 

 families supplied were invaded ; in typhoid fever 59 per cent, were 

 invaded ; and in diphtheria outbreaks 32 per cent The average 

 percentages may be tabulated as follows : — 



These figures raise various points of interest, into which we 

 cannot now enter. Briefly, it may be said that the chief reasons 

 of the comparatively low percentages of families invaded out 

 of families supplied are social and economical. Dairj-men have 

 various supplies, some of which are mixed, and some of which are 

 unmixed, and various persons handle the milk. Hence, though a 

 milkman supplies one hundred families, only forty-nine, or fifty- 

 nine, or thirty-two become affected with the disease, partly owing 

 to distribution, and partly owing to conditions of domestic economy. 



The subject of distribution is an important one. It is a general 

 custom for purveyors of milk to obtain their daily supply, not from 

 a single source, either farm or dealer, but from several. Hence 

 it may come about that a milkman supplies, say, one hundred 

 customers, and yet only a dozen contract infectious disease. From 

 this it is argued that milk cannot have conveyed the disease, for if 

 it had done so, a higher percentage would have been attacked. Of 

 course the fallacy is obvious. To arrive at a correct conclusion it 

 is necessary to trace each source of the milkman's supply, and its 

 particular distribution. This factor will frequently be found to 

 be the explanation of a low percentage of families invaded. In 

 regard to individuals attacked, idiosyncrasy and individual powers 

 of resistance, of course, also play their part. 



(d) Age and sex incidence.— It generally happens that women 

 and children are the heaviest milk-drinkers, and for this reason it 



follows that the female sex and children of tender years are more 

 affected in milk epidemics than are men. For some time in the 

 earlier investigations of milk-borne disease this general principle 

 was so strongly held that if an outbreak occurred including a large 



