374 PATHOGENIC MICROORGANISMS 



comprehension of infectious disease as it occurs in man, is a funda- 

 mental biological knowledge of the bacteria themselves, their actions 

 upon artificial media, the characteristics by which they can be dif- 

 ferentiated, the conditions under which they grow and produce 

 poisons, and the reactions which they or their poisons elicit in 

 animals. Of course there are many points in such a chain of reason- 

 ing which investigation has not yet cleared up, and there is, 

 especially, much uncertainty and half-knowledge in regard to some 

 of the most important phases of the chemical and immunological 

 reactions which take place between the invaders and the animal 

 body. Moreover, it is quite likely that many of our theories con- 

 cerning the fundamental principles which govern such reactions are 

 defective. But without, at least, a knowledge of the biological facts 

 available, the physician who is confronted by a human infection is 

 working very largely in the dark. It is the task of the specialized 

 laboratory worker to prepare this material and submit it to the 

 clinician so that we may use it in the premises of his reasoning. 



Considered in this way, an infection in an animal or a human 

 being resolves itself into a balance between the forces of infection, 

 on the one hand, and the injuries and resisting mechanisms of the 

 infected subject, on the other. 



We may consider the entrance of a foreign living being into the 

 tissues of a higher animal or plant as a process in which a struggle 

 for existence is initiated. The invading microorganism must take 

 its nourishment from the invaded host, abstracting thereby materials 

 needed by the host, and, in the course of its multiplication and 

 digestive processes, it not only injures the host mechanically by 

 local accumulation, but also by the remote action of the substances 

 which it produces in the course of its metabolism, many of which 

 are toxic. The host, if not overwhelmed, responds by reactions 

 which express themselves both in local morphological changes in 

 places where direct contact with the bacteria is established, and by 

 remote systemic reactions incited by absorption not only of the toxic 

 derivatives of the bacteria, but also to some extent of the products 

 of the local struggle in which proteolytic destruction, necrosis, etc., 

 are involved. The symptom complex, therefore, which we recognize 

 as disease results in part from injury, but to a larger extent repre- 

 sents the manifestations of the defense reactions of the host. When 

 a physician makes a diagnosis of a particular infection by "history 

 and physical examination, he does so on the basis of his own observa- 



