DISEASES CAUSED BY INFECTED MILK 91 



volve a thousand or more. They are perhaps more likely 

 to occur In the summer-time when the milk is warm and 

 the infection has a chance to grow. Milk-borne epidemics, 

 however, have been traced to milk in the winter-time, and 

 also to milk which has been kept on ice until it reached the 

 consumer. Cold, therefore, is a preservative rather than a 

 preventive against these dangers. Even freezing does not 

 kill these infections. 



Milk-borne outbreaks usually have an explosive onset. 

 The disease follows the milk route. In fact, the disease 

 may occur only among users of the infected milk. When 

 the cases are charted upon a map the milk routes re- 

 semble thoroughfares of infection but, for several reasons, 

 the disease is not always limited strictly to those who use 

 the milk. The first is that a milk-borne outbreak usually 

 occurs when the disease prevails in the neighborhood. In 

 fact, the greater the prevalence of the infection the greater 

 are the mathematical chances of contaminating the milk. 

 Further, secondary cases soon occur, for each person is a 

 focus from which the disease spreads. There is, however, a 

 special incidence of the disease in milk drinkers. For in- 

 stance, the only member of a family or the only person in a 

 large boarding-house to be attacked will be a person drink- 

 ing the raw milk; on the other hand, the only person exempt 

 will be the sole one not using it. It is not only persons fond 

 of milk who are apt to be attacked, for milk products also 

 may convey the disease. Thus disease may be contracted 

 simply by the use of a little cream in coflFee or upon cereals, 

 or the drinking of soda water containing cream, the eating 

 of ice cream, etc. 



People in the better walks of life are often attacked in 

 greater proportion than others. This is explained by the 

 fact that families with larger incomes are supposed to 

 drink more milk than those with lesser resources. Among 

 the well-to-do, therefore, it frequently happens that in- 

 fected milk finds more victims, while among the poor the 



