ACUTE INFECTIOUS DISEASES 73 



vessels to be washed in water which is certainly liable to 

 sewage pollution. Cows frequently have to drink from sewage- 

 polluted sources, and contaminate their udders by standing 

 up in such water. 



Infection of Bovine Origin. — The possibility of this source 

 of infection is considered in detail in Chapter VI. 



The General Characters of Milk-borne Outbreaks 



While variations are not uncommon, certain broad general 

 features are met with, and may be looked for in outbreaks 

 spread by milk. The following are the most important : — 



1.. The Incidence is upon those ivho drink a Particular 

 Supply of Milk. — The invaded houses have a common milk 

 supply, and usually nothing else in common. Inquiry shows 

 that the households supplied with milk from a particular 

 source are picked out and infected. While many such house- 

 holds escape, it is found that when comparative inquiries are 

 made the consumers of the implicated supply furnish a much 

 higher percentage of cases than either the rest of the com- 

 munity or the consumers of milk from any other source. 



The percentage affected of consumers of the implicated 

 milk varies greatly in different epidemics and for epidemics of 

 different diseases. Variations as wide apart as 4 and 100 per 

 cent have been recorded. 



The smallest percentage invasion of households is met with 

 in scarlet fever outbreaks. It would probably be higher if all 

 ill-defined cases with symptoms mainly or exclusively those 

 of sore-throat were notified and included. 



That only a portion, and that a variable one, of the con- 

 sumers of the implicated supply become infected is, of course, 

 readily explainable. In many households the milk would be 

 rendered harmless by boiling or cooking before use ; in others, 

 the particular milk delivered might be free from the infecting 

 organisms although part of the same general supply ; in others, 

 the consumers might be insusceptible from natural or acquired 

 immunity. In many, also, the infected milk might be con- 

 sumed, but infection not occur owing to the organisms being 

 killed in the body before they had opportunity to infect. 

 In other words, when the difficulties of individual infection 



