CHAPTER I. 



INFECTIONS 



By the term infection we understand the entrance of micro- 

 parasitic living agents into the body tissue or substance, and 

 the occurence of definite symptoms of disease as the result of 

 the multiplication and action of the invading organisms. Infec- 

 tions do not always occur when pathogenic microorganisms are 

 present in the body tissues, certain conditions being necessary 

 for the establishment of a bacterial disease. These conditions 

 may be broadly divided into two classes; those dependent upon 

 the biological properties of the infecting organism, and those 

 dependent upon the conditions in the host and tissues invaded. 



Of the biological characters of importance in the infecting 

 organism in the production of a bacterial disease, the most im- 

 portant are, that the microorganisms must be able to multiply 

 rapidly and greatly in the tissues of the body, and that they must 

 be able to produce poisons or substances harmful, either to some 

 or to all, of the tissues of the body. The number of organisms 

 necessary to cause disease varies with different species. Many 

 more staphylococci are necessary to cause the formation of a 

 furuncle than of anthrax bacilli to cause anthrax. The property 

 to produce substances poisonous to the body tissues generally 

 determines virulence. That virulence is of great importance is 

 evident from the fact that innumerable so-called saprophytic 

 bacteria which are present in the different parts of the body, grow 

 and multiply there but because in their growth sufficient poison 

 is not produced, cannot cause an infection. The virulence of an 

 organism, according to numerous investigators, depends upon 

 the presence of certain substances in the parasite which reduce 

 the resistance of the body and its tissues. These substances are 

 variously designated as lysins and aggressins. 



