BACTERIA IN FOODS 197 



the presence of the bacillus of tubercle, which make the dis- 

 ease common amongst cattle. Constitution, temperament, 

 age, work, food, and prolonged lactation are the individual 

 features which act as predisposing conditions; they may act 

 by favouring the propagation of the bacillus or by weaken- 

 ing the resistance of the tissues. To this category must 

 further be added conditions of environment. Bad stabling, 

 dark, ill-ventilated stalls, high temperature, prolonged and 

 close contact with other cows, all tend in the same direction. 



Though there can be no doubt as to the virulence of tuber- 

 culous milk, it may be remembered with satisfaction that 

 only about two per cent, of tuberculous cows have unmis- 

 takably tubercular milk. Even of this tubercular milk, 

 unless it is very rich in bacilli and is ingested in large quan- 

 tities, the risks are practically small or even absent. Practi- 

 cally the danger from drinking raw milk exists only for 

 persons who use it as their sole or principal food, that is to 

 say, young children and certain invalids. With adults in 

 normal health the danger is greatly minimised, as the healthy 

 digestive tract is relatively insusceptible. Moreover, dairy 

 milk is almost invariably mixed milk; that is to say, if 

 there is a tubercular cow in a herd yielding tubercle bacilli 

 in her milk, the addition of the milk of the rest of the herd 

 so effectually dilutes the whole as to render it almost 

 innocuous. 



But if for practical purposes we look upon all milk derived 

 from tubercular udders as highly infective, we may adopt a 

 comparatively simple and efficient remedy. To avoid all 

 danger it is sufficient to bring the milk to a boil for a few 

 minutes before it is consumed ; in fact, the temperature of 

 85 C. (160 F.) prolonged for five minutes kills all bacilli. 

 The common idea that boiled milk is indigestible, and 

 that the boiling causes it to lose much of its nutritive value, 

 is largely groundless. 



Milk may become tubercular through the carelessness or 



