VIRULENCE 95 



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 

 toxin produced by one species is different from that produced by another 

 in the kind of disease produced and the species of animal infected. 

 Some toxins are active for certain animals only and not for others. 

 Microorganisms of one group may possess general and common patho- 

 genic properties differing only in degree; those of different morphologic 

 and cultural characters may possess totally different powers. 



The virulence of a given species is subject to great variation. A few 

 bacteria almost constantly retain their virulence, even when kept for 

 years under artificial conditions; as an example may be mentioned the 

 diphtheria bacillus; others quickly lose their virulence as soon as they 

 are grown artificially, as, e. g., the influenza bacillus; in others and 

 probably the larger class the virulence may be raised or lowered ac- 

 cording to the experimental manipulations to which they may be sub- 

 jected. Variations may also be observed among members of the same 

 group of microorganisms, and even among individual microorganisms 

 of the same strain. 



Decrease of virulence of a microorganism may be brought about 

 artificially by repeated growth in pr upon culture-media, especially 



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