218 DISEASES FROM HARMFUL ORGANISMS 



should because of its common occurrence and compara- 

 tively mild ill-effects, but there can be no doubt that 

 human efficiency is definitely lowered as a result of this 

 kind of poisoning. 



The Metabolic Activities of Parasitic Organisms. 

 — In previous paragraphs it has been shown that organ- 

 isms occurring in foods or within the alimentary tract may 

 produce poisons in connection with their metabolic activi- 

 ties. It is also true that certain kinds of organisms may 

 live on or within the body and produce poisons by which 

 the body may be harmed. These make up the group of 

 so-called disease-producing organisms. It should be noted 

 that the distinction between disease-producing organisms 

 and such organisms as do harm through causing food 

 spoilage or intestinal putrefaction depends on whether 

 they affect the body directly or indirectly. The true 

 disease-producing organism is one which is parasitic upon 

 or within the body and liberates its poisons directly in 

 the cells or body fluids. The presence of these or- 

 ganisms upon or in the body in numbers sufficient to 

 cause harm constitutes what is technically known as 

 an injection. 



The Relation of Host and Parasite is a maladjust- 

 ment of association. Infection from the purely biologi- 

 cal point of view is an incident in the struggle for exist- 

 ence. Parasitic organisms must establish themselves in 

 certain relations wdth host organisms or they will perish. 

 The ideal relation of parasite with host is obviously one 

 in which both parasite and host flourish, i.e., symbiosis,. 

 for if the parasite destroys the host it presently finds 

 itself deprived of its source of nourishment and must find 

 a new host, or die. Infections that injure the host are, 

 from this point of view, maladjustments in the associa- 

 tions of organisms. 



Classes of Disease-Producing Organisms. — Three 

 classes of organisms are recognized as capable of produc- 

 ing infection ; those which are undoubtedly plants, those 



