CHAPTER V. 



SOURCES OF PATHOGENIC MICRO-ORGANISMS 



(Continued.) 

 (5) From Man to Man. 



As indicated in Chapter I. there are all degrees 

 in the facility with which infectious diseases are 

 transmitted from one person to another, varying 

 from that in which it is only necessary to breathe 

 the air surrounding a patient (scarlet fever, 

 measles, influenza, etc.), to that in which trans- 

 mission never takes place under ordinary circum- 

 stances (tetanus, hydrophobia). 



In some instances a logical relationship exists Relation of 

 between the facility of transmission, on the one MetiTod of 

 hand, and the form of excretion of the micro-or- Excretl 

 ganisms and their preferred infection atrium, on 

 the other. Thus in many diseases in which the 

 micro-organisms are excreted from the respiratory 

 passages the latter are also used as the infection 

 atrium (influenza, measles, smallpox, etc.). In 

 others, in which they are excreted mainly by the 

 intestines, the infection atrium is the intestines 

 (typhoid, cholera, dysentery). Even if it were 

 possible for the organisms of typhoid and cholera 

 to gain entrance through the lungs (and indeed it 

 may be possible in typhoid at least), the fact of 

 their excretion mainly by the stools, and by the 

 urine in typhoid, would render primary pulmonary 

 involvement difficult. They reach the intestines 

 more readily through contaminated food, water, 



