22 INFECTION AND RESISTANCE 



difficult. However, the recent work of Rosenow 41 on pneumonococci 

 seems to bring some reinforcement to the ranks of those who main- 

 tain the existence of a special offensive substance at the command of 

 virulent bacteria. Rosenow extracted pneumococci grown on serum 

 broth and found that such extracts when made from virulent strains 

 would protect avirulent strains from engulfment by phagocytes. The 

 non-virulent strains left in these extracts for 24 hours became viru- 

 lent. He believes, therefore, that the virulence of pneumococci de- 

 pends largely upon the possession of these substances which he calls 

 "viruiins," and which in function at least are conceived as very 

 similar to the "aggressins." 



Recent results obtained by the writer 42 with Dwyer seem to 

 indicate that anaphylatoxins produced from the typhoid bacillus 

 possess some of the properties claimed for his aggressin by Bail. 

 It is not impossible that the "aggressins" obtained by him were of 

 this nature. 



Virulence, then, may be analyzed into two main attributes : one a 

 purely passive property of resistance or self-preservation on the 

 part of the bacteria, perhaps morphologically expressed in ectoplas- 

 mic hypertrophy and capsule formation; the other an actively of- 

 fensive weapon in the form of substances of the nature of the "ag- 

 gressins" of Bail or the "virulins" of Rosenow. The extent of our 

 present knowledge of details does not warrant a statement of the 

 case in more definite terms. 



From the facts we have discussed in the preceding paragraphs 

 it now becomes manifest that the elements which determine the 

 nature of an infectious disease are twofold. On the one hand 

 each variety of infectious germs possesses certain biological and 

 chemical attributes which are specific and peculiar to itself ; by these 

 its predilection for path of entrance and mode of attack is de- 

 termined, and upon these depends the nature of the reaction called 

 forth in the animal body. On the other hand the degree of infec- 

 tion in each case, the severity of the reaction and the ultimate out- 

 come are determined by the balance which is struck between the 

 virulence of the entering germ and the protective mechanism opposed 

 to it. 



The specific properties of each micro-organism are the factors 

 which account for the clinical uniformity (within definite limits) 

 which is observed in the maladies produced in different individuals 

 by the same species of bacteria. Thus a severe typhoid fever is, in 

 essential characteristics, entirely similar to a mild case since in 

 both instances the path of entrance, through the intestine, is the 

 same, the distribution of the germs after entrance differs only in 

 degree, and the reactions, local and systemic, which are called forth 



41 Rosenow. Jour, of Inf. Dis., Vol. 4, 1907. 



42 Zinsser and Dwyer. Proc. Soc. Exp. Biol. and Med., Feb., 1914. 



