FILTH AS infants' FOOD 93 



have identified a certain number of varieties of bac- 

 teria and classified them as being pathogenic, and why 

 they believe many other varieties to be harmless and 

 incapable of causing disease, having observed them 

 very closely and in large numbers without detecting 

 any ill effects from their presence. It will be under- 

 stood, too, why it is that they are uncertain about 

 some other varieties and do not yet venture to classify 

 them as being either pathogenic or harmless. Some 

 of the harmless varieties, it should be observed, are 

 only harmless from a pathogenic standpoint. They 

 do not cause disease, but they are very troublesome in 

 many ways. They cause milk to turn sour, for exam- 

 ple, inflicting quite severe losses upon the farmer or 

 milk vendor sometimes. We are apt to blame the 

 souring of the milk upon the weather, particularly 

 if there has been a thunderstorm, rather than the real 

 culprit, the little micro-organism in the milk. Man 

 has even found some of the bacteria useful and cun- 

 ningly availed himself of the fruits of their labor, for 

 the various kinds of cheese which we consume are all 

 produced from milk, partly by bacterial action. Cul- 

 tures of bacteria for cheese-making are now regularly 

 sold on the market." 



But how, it may be asked, is it possible for bacteria 

 to get into the milk in large numbers? To answer 

 that question it will be necessary to describe the pro- 

 cess of milking the cow, and the transportation of the 



