182 INFECTION AND IMMUNITY 



exert their harmful action chiefly by local growth and the elaboration of 

 specific poisons. 



The inciting or inhibiting factors which permit or prohibit an in- 

 fection are dependent iri part upon the nature of the invading germ and 

 in part upon the conditions of the defensive mechanism of the subject 

 attacked. 



Bacteria are roughly divided into two classes, saprophytes and 

 parasites. The saprophytes are those bacteria which thrive best on 

 dead organic matter and fulfill the enormously important function in 

 nature of reducing by their physiological activities the excreta and 

 dead bodies of more highly organized forms into those simple chemical 

 substances which may again be utilized by the plants in their con- 

 structive processes. The saprophytes, thus, are of extreme importance 

 in maintaining the chemical balance between the animal and plant 

 kingdoms. Parasites, on the other hand, find the most favorable 

 conditions for their development upon the living bodies of higher forms. 



While a strict separation of the two divisions can not be made, nu- 

 merous species forming transitions between the two, it may be said 

 that the latter class comprises most of the so-called pathogenic or 

 disease-producing bacteria. Strict saprophytes may cause disease, 

 but only in cases where other factors have brought about the death 

 of some part of the tissues, and the bacteria invade the necrotic 

 areas and break down the proteids into poisonous chemical sub- 

 stances such as ptomains, or through their own destruction give 

 rise to the liberation of toxic constituents of their bodies. It is 

 necessary, therefore, that bacteria, in order to incite disease, 

 should belong strictly or facultatively to the class known as para- 

 sitic. It must not be forgotten, however, that the terms are relative, 

 and that bacteria ordinarily saprophytic may develop parasitic 

 and pathogenic powers when the resisting forces of the invaded 

 subject are reduced to a minimum by chronic constitutional disease 

 or other causes. 



Organisms that are parasitic, however, are not necessarily pathogenic, 

 and there are certain more or less fundamental requirements which 

 experience has taught us must be met by an organism in order that it 

 may be infectious (or pathogenic) for any given animal; and by infec- 

 tiousness is meant the ability of an organism to live and multiply in the 

 animal fluids and tissues. For instance, an organism which is shown 

 not to grow at the body temperature of warm-blooded animals may 

 safely be assumed not to be infectious for such animals ; and experience is 



