INFECTION 31 



body are similar to those produced by drugs and can be 

 reproduced by the injection of bacterial poisons. 



Infection is the successful invasion of an organism by 

 microparasites, and implies an abnormal state resulting from 

 the deleterious action of the parasite upon the host. 



Sources of infection may be exogenous or endogenous. 

 Exogenous infections result from the successful invasion of 

 the body by microparasites from sources entirely apart from 

 the individual infected. Infection by the typhoid bacillus 

 from water or milk, by the Spirochaeta pallidum from dental 

 instruments or drinking-cups, contraction of smallpox from 

 fomites, and contraction of malaria from the bites of mosqui- 

 tos are examples of exogenous infection. 



Endogenous infections result from the successful invasion 

 of the body by microorganisms normally present on the 

 body. The skin and mucous membranes furnish lodgment 

 for a great variety of virulent pathogenic organisms which, 

 when the resistance of the body is lowered, immediately 

 become invasive. The pneumococcus is a normal inhabitant 

 of the mouth and pharynx, but causes no infection until the 

 body resistance is lowered. When this occurs, tonsillitis, 

 pharyngitis, or lobar pneumonia may follow. 



Pathogenesis. The ability of a microorganism to do 

 harm depends on its invasive powers and its ability to gener- 

 ate toxins or both. 



Toxins. Little is known of the chemic nature of toxins. 

 Undoubtedly some are related to albumins. Others give no 

 reactions common to compounds of this group. 



(A) Intracellular or Insoluble Toxins. These are chiefly 

 within the bodies of the bacteria, and are set free by disin- 

 tegration of the organism. This group comprises most of 

 the pathogenic bacteria. 



(B) Extracellular or Soluble Toxins. These toxins are ap- 

 parently excreted by the bacteria, and are found in the 

 surrounding medium. This group includes the diphtheria 

 and tetanus bacilli. 



It has been shown that bacteria which apparently do not 



