SUSCEPTIBILITY AND INFECTION 71 



to infection, as is shown by the frequent presence upon 

 them of virulent germs, as the diphtheria and influenza 

 bacillus and the streptococcus in persons in perfect health. 

 The constant flow of mucus and the movement of the 

 ciliated epithelium tend to prevent the entrance of the 

 germs through these surfaces, but injury, excessive dry- 

 ing of the surfaces from overheated rooms, and disease 

 may lower the resistance offered by these surfaces and 

 increase the liability to infection. 



Few bacteria reach the lungs, but those that do are 

 quickly taken up by the fixed epithelial cells and the 

 leukocytes. Under certain unfavorable conditions this 

 does not occur, and infection occurs through the lungs. 

 The stomach is exceptionally free from infection. This 

 is doubtless due to the fact that the gastric juice is detri- 

 mental to bacterial activity. It is not germicidal, how- 

 ever, and therefore does not prevent the entrance into 

 the intestines of bacteria, which may there cause 

 trouble. 



The intestines present a feeble resistance to bacterial 

 activity in the bile, but, on the whole, bacteria do fairly 

 well in the intestinal contents. The intestinal walls 

 normally, however, present considerable resistance to the 

 entrance of germs, as is evidenced by the large number 

 of bacteria found in the intestinal contents of healthy 

 individuals, even typhoid bacilli and cholera germs hav- 

 ing been found in the bowel movements of healthy 

 persons. Certain conditions, as improper feeding, 



