134 THE STORY OF GERM LIFE. 



if they could be traced to their source would be 

 found to be produced by bacterial poisons swal- 

 lowed with food or drink, or by similar poisons 

 produced by bacteria growing in such food after 

 it is swallowed by the individual. In hot weather, 

 when bacteria are so abundant everywhere and 

 growing so rapidly, it is impossible to avoid such 

 dangers completely without exercising over all 

 food a guard which would be decidedly oppress- 

 ive. It is well to bear in mind, however, that 

 the most common and most dangerous source of 

 such poisons is milk or its products, and for this 

 reason one should hesitate to drink milk in hot 

 weather unless it is either quite fresh or has been 

 boiled to destroy its bacteria. 



PATHOGENIC BACTERIA WHICH ARE TRUE 

 PARASITES. 



This class of pathogenic bacteria includes 

 those which actually invade the body and feed 

 upon its tissues instead of living simply upon 

 swallowed food. It is difficult, however, to draw 



any sharp line sep- 

 arating the two 

 classes. The bac- 

 teria which cause 

 diphtheria (Fig. 

 28), for instance, 

 do not really in- 



FIG. 28. Diphtheria bacillus. vade the body. 



They grow in the 



throat, attached to its walls, and are confined to 

 this external location or to the superficial tissues. 

 This bacillus is, in short, only found in the mouth 

 and throat, and is practically confined to the so- 



