94 INFECTION 



the toxic products, as when portions of placenta or fetal membranes 

 are retained in the uterus after childbirth. 



The term toxemia is employed rather loosely to mean the presence 

 of any toxic material. Its use should be limited to the condition re- 

 sulting from the absorption of the poisonous substances produced by the 

 non-invasive bacteria themselves, as in diphtheria and tetanus. Septi- 

 cemia is the term applied to the presence in the body-fluids of toxic products 

 generated by the pyogenic microorganisms. 



MECHANISM OF INFECTION 



Since bacterial invasion is of frequent occurrence, the question 

 naturally arises, Why are not infections, both local and general, more 

 frequent? Thus abrasions of the surface epithelium are not uncommon 

 in the presence of active microorganisms; tubercle bacilli may be in- 

 spired, and typhoid bacilli may be swallowed, the altered local con- 

 ditions affording opportunity for producing infection, and yet the host 

 may escape. 



Bacterial invasion, therefore, does not necessarily mean infection, and 

 it may be stated that infection can only take place when 



(1) The microorganisms are sufficiently virulent. 



(2) When they invade the body by appropriate avenues and reach 

 susceptible tissues. 



(3) When they are present in sufficient numbers. 



(4) When the host is generally susceptible to their action. 



(5) When the microorganisms are able to resist the defensive forces 

 of the host through special agencies aside from their offensive forces. 



Not all these factors must necessarily be present before infection may 

 occur. A microorganism may be particularly virulent, so that numbers 

 are relatively unimportant; a host or a portion of the host may be so 

 susceptible or vulnerable to infection that a microorganism of low 

 virulence, which, under normal conditions, would be totally unable to 

 produce infection, may now prove pathogenic. 



VIRULENCE 



Virulence refers to the disease-producing power of a microorganism, 

 and is dependent upon two variable factors: (1) Toxicity, and (2) aggres- 

 siveness, or the invasive power of the bacteria. In most infections usually 

 both factors are operative. 



Toxicity is the term applied to the kind and amount of poison or toxin 

 produced. This poison may be readily soluble, or exogenous, diffusing 





