VIRULENCE 95 



(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 

 into the surrounding tissues and being readily absorbable; or it may be 

 endogenous, and contained chiefly within the microorganisms, and be 

 liberated only upon the dissolution of the bacterial cell. 



Aggressiveness is a term applied to the invasive powers of a micro- 

 organism to enter, live, and multiply in the body-fluids, or, in other words, 

 to the aggressive or progressive forces of the microorganism in its new 

 environment. 



Toxicity is generally confused with aggressiveness, a highly toxic 

 microorganism being regarded as an aggressive one. For example, the 

 bacillus of tetanus is highly toxic because of the production of a potent 

 soluble poison which gives rise to the symptoms of tetanus, although 

 it is only slightly aggressive, being almost unable to multiply in the 

 tissues. The anthrax bacillus, on the other hand, is highly aggressive, 

 owing to the fact that it usually multiplies to such an extent that it can 

 be found in each drop of blood and in every organ of an infected animal; 

 nevertheless it is but slightly toxic, the animals frequently showing few 

 or no symptoms until shortly before death. The toxicity of a micro- 

 organism should, therefore, be regarded separate from its aggressive- 

 ness, although in many infections both factors are so intimately con- 

 cerned that the term virulence may be used to express the degree of 

 pathogenicity or the total disease-producing power. 



The virulence of a microorganism is more or less specific, i. e., the 



