THE PROBLEM OF VIRULENCE 11 



sparsely or not at all unless definite conditions of nutrient materials, 

 temperature, reaction, and osmotic pressure are observed. On the 

 other hand, typhoid, anthrax, and dysentery bacilli, staphylococci 

 and numerous other pathogenic germs grow easily and luxuriantly 

 on the simplest laboratory media and within a wide range of envi- 

 ronmental variations. 



Biologically considered, we could arrange the scale of adaptation 

 to parasitic conditions on this basis and it would seem, a priori, that 

 those bacteria which had thus adapted themselves most closely to the 

 living body should be the most infectious. There is not, however, 

 such parallelism, since many of the most powerfully invasive or 

 virulent germs, for instance, the anthrax bacillus, have retained 

 their capacity for saprophytic life to the fullest extent. It is more 

 logical, therefore, to classify parasites, not according to their ability 

 to revert to saprophytic conditions, but rather, as Bail 13 has done it, 

 on the basis of their relative powers of invading the living body. 

 His classification, of course, implies that the position of each micro- 

 organism in this scale must be determined with reference to a given 

 animal species, since a germ which is highly infectious ("parasitic" 

 in Bail's sense) for one species may be a "half-parasite" or even a 

 pure saprophyte for another. 



Briefly reviewed, his classification is as follows: 



I. Pure Saprophytes. (Xecroparasites, superficial parasites, or 

 external parasites.) 



Micro-organisms which under no circumstances can be made to 

 develop within the living tissues of a given animal. This does not 

 exclude their pathogenicity for this animal, since, like the diphtheria 

 or tetanus bacillus, they may develop and produce toxins on the 

 basis of a localized area of dead tissues. 



II. Pure Parasites. Organisms like the anthrax bacillus or 

 the bacilli of the hemorrhagic septicemia group which, implanted in 

 small quantity in an animal, will rapidly gain a foothold, thrive, 

 and spread throughout the body. 



III. Half parasites, organisms which may be infectious if in- 

 troduced into the animal body, but, not possessing this invasive 

 power to the same degree as the preceding class, require the inocula- 

 tion of considerable quantities, often a special mode or path of in- 

 oculation, or even possibly a preliminary reduction of the local and 

 general resistance of the infected individual in order that they may 

 multiply and become generalized. This class includes the large 

 majority of the bacteria pathogenic for man. 



This property of invasive power is spoken of as virulence in 

 contradistinction to toxicity the latter implying merely the abil- 

 ity to produce poisons, and not necessarily being associated with the 

 power to invade. 

 13 Bail. Loc. cit. 



