Disease Transmission and Mans Resistance 303 



of acid found in gastric juice probably is detrimental to some or- 

 ganisms, and some of the enzymes undoubtedly affect bacteria if 

 food remains in the stomach long enough for such enzyme activity 

 to take place. Liquids tend to pass through the stomach into the 

 intestines rather quickly, and organisms are carried along with the 

 tide of fluid which, incidentally, temporarily dilutes the acid in the 

 stomach. 



Mucous Membranes 



The moist membranes lining the nose, mouth, and nasopharynx 

 serve a useful purpose as part of our defense mechanism. With 

 the aid of cilia which sweep trapped material up toward the 

 mouth, and the protective sieve afforded by the hairs of the nose, 

 microorganisms have a relatively difficult time getting into the 

 deeper respiratory passages. Since people do contract pneumonia, 

 however, it is evident that the more aggressive organisms can 

 overcome the body defenses, particularly when the resistance of 

 the host is reduced. 



The Lungs 



These organs are constantly bathed with body fluids from their 

 extensive blood supply. The many barriers between the hairy 

 moist surface of the nose and the rather distantly removed lungs, 

 help to minimize the number of organisms which find their way to 

 the lungs. Once bacteria do arrive at the lungs, various physio- 

 logical forces help to destroy them. 



The Intestines 



After food leaves the stomach and enters the upper intestines, 

 a pH reversal occurs, and at the point where bile flows into the 

 intestines, marked changes in the bacterial flora take place. Prac- 

 tically a pure culture of Escherichia coli can be found just below 

 this bile inlet. Enteric pathogens, including the typhoid and 

 paratyphoids, must be able to survive the action of stomach juices, 

 bile, and enzyme activity in the intestines in order to cause disease 

 in the digestive tract. 



