CHAPTER XLII 

 A THEORY OF MICROBIC VIRULENCE 



I. S. FALK 



University of Chicago 



INTRODUCTORY 



The term "virulence" is used sometimes loosely to denote the absolute capacity 

 of a parasite to cause disease. On other occasions, when suitably qualified, it is used 

 to describe the severity or lethality of the disease which is induced by a parasite. In 

 almost every case, however, "virulence" refers primarily to a characteristic of the 

 parasite, even as "resistance" or "immunity" denotes a state of the host. For reasons 

 which will be discussed at length, such procedures must be considered unsound. 



In any circumscribed series of comparable observations, virulence is never known 

 and probably cannot be known except as a reciprocal measure of resistance. The host 

 for which a particular microbe displays virulence is relatively non-resistant; and the 

 microbe to which a particular host shows high susceptibility is virulent. Such state- 

 ments as these may seem gratuitous; but they are necessitated by the common, per- 

 haps inadvertent, reference to virulence and resistance as though they were absolute 

 characteristics of parasite and host, respectively. The only precise definition is the 

 following : 



Virulence cc 



Resistance (or Immunity) ' 



i.e., virulence varies reciprocally as resistance or immunity. According to the precision 

 with which virulence is described in terms of the experimental conditions under which 

 it is measured, it takes on proportionally absolute characteristics. A discussion of 

 \'irulence is then resolved into an analysis of the variables which, by operating pri- 

 marily upon the parasite, whether in vivo or in vitro, affect the course or the outcome 

 of a particular infection. Correspondingly, immunology is concerned with the anal- 

 ysis, preferably the simultaneous analysis, of resistance. 



It is apparent that every precise measurement of virulence, whether expressed in 

 terms of the number of organisms in a minimal infecting or fatal dose, in terms of the 

 size of a lesion produced, or of the elapsed time between infection and the develop- 

 ment of a particular diseased state or of death, must always be qualified by the his- 

 tory of the host as well as of the parasite. For a particular host virulence is variable 

 not only for different parasites, or different members of a single genus or a particular 

 species, but even for different strains of the same variety and for the same strain 

 studied at different times, in various sul^cultures or after purposive modification by 

 experimental procedures. 



When variations in virulence occur spontaneously, they must be taken as indica- 

 tive either of genetic instability in the parasite or of failure to recognize and to hold 



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