DISSEMINATION OF ORGANISMS. 19 



transmission is given in order to illustrate the 

 idea and limits of contagiousness. 



1. Those which are characterized bv ready trans- Facility and 



. . mi ' J . Means of 



mission through the air. The micro-organisms are Transmission. 

 discharged into the air from the respiratory 

 passages, or from the skin in desquamative infec- 

 tions, and it is onry necessary for a susceptible 

 person to come within the zone of infected air 

 which surrounds the patient in order to acquire 

 the disease. Actual contact with the patient may 

 facilitate transmission in some instances, but is 

 not necessary, and many of them are transmitted 

 by indirect contact, i. e., through the agency of 

 intermediate persons, through food contamination 

 (scarlet fever in milk), or through inanimate sub- 

 stances which have been in contact with the pa- 

 tient. These are all highly contagious "air-borne" 

 diseases, which probably use the respiratory pas- 

 sages as their infection atrium. Diphtheria, scar- 

 let fever, measles, rotheln, smallpox, influenza, tu- 

 berculosis, and plague in plague pneumonia are of 

 this type. 



2. Transmission occurs almost exclusively by Personal 

 personal contact, and usually a special form of 

 contact, never through the air : gonorrhea, syph- 

 ilis, soft chancre, and perhaps dourine in horses. 

 Syphilis occasionally is transmitted by indirect 

 contact, the free period being a short one. These 



are contagious diseases. 



3. Transmission is chiefly by indirect contact, indirect 



V.. J . Contact. 



or through lood or water. .Direct contact plays a 

 role in some instances, and rarely the air may be 

 a means of conveyance. The members of this 

 group are not highly contagious in the sense of 

 transmission directly from one individual to anoth- 



