THE RELATIONS OF BACTERIA TO DISEASE 51 



contains large numbers of bacteria of all kinds, - 

 the bacilli of diphtheria, of influenza, and the 

 pneumococci are often present, but the saliva has 

 a slight germicidal power and serves as a constant 

 wash to the membranes of the mouth; the hairs 

 of the nasal and aural passages and the eyelashes 

 make some obstruction to the entrance of bacteria ; 

 and the moist surfaces of the air-passages retain 

 most of the bacteria contained in the air before it 

 reaches the lungs. The hydrochloric acid of the 

 gastric juice is unfavorable for the growth of bacteria, 

 although it does not always kill them ; the bacillus 

 of typhoid fever, for instance, passes through the 

 stomach and finds favorable conditions for develop- 

 ment in the intestines. 



Large numbers of bacteria are found in the intes- 

 tines of all perfectly healthy persons, but unless 

 resistance is lowered from some cause, no manifesta- 

 tion of disease occurs. 



Transmission of Disease. Infectious diseases 

 are transferred either directly (contact) from a 

 patient to another individual, or indirectly through 

 air, water, food, or carried by insects. 



As a rule, the pathogenic bacteria are incapable of 

 living except for a very short time apart from the 

 body, but the fact that they may exist for a con- 



