72 VARIATIONS IN VIRULENCE [CH. vi 



substances assimilated and excreted by them ; (b) the dis- 

 integration of the dead organisms ; (c) the mechanical effect 

 of their presence whether living or dead ; (d) the response 

 made by the living tissues to these various stimuli. 



The first factor in the production of disease the "in- 

 vasiveness " of the organism is dependent upon the degree 

 of "mobility" the particular organism possesses under various 

 conditions. 



The second factor the result of their activity within the 

 body may be considered under two heads : (1) the produc- 

 tion of particular lesions in the tissues and the exhibition 

 of a certain train of symptoms, both more or less peculiar to 

 the particular organism giving rise to them phenomena which 

 are discussed under the term " pathogenesis," and (2) the 

 production of a condition of "toxaemia," resulting in a 

 general impairment of health and leading eventually to the 

 death of the body, which we now propose to consider under 

 the term "virulence." 



The virulence of organisms is known to vary under different 

 circumstances within wide limits. 



1. Thus, cases of disease which occur at the end of an 

 epidemic are frequently, though not invariably, less severe 

 than those at the beginning, the virulence of the infecting 

 organism having gradually diminished during the course of 

 the epidemic. Thomson mentions an epidemic of cerebro- 

 spinal fever comprising 30 cases, all of which were admitted 

 to hospital and received the same treatment ; of the first 16 

 cases admitted all died except two, of the last 14 admitted all 

 lived except two. 



This difference in virulence might of course be apparent 

 and not real, the lessened intensity of the disease being 

 accounted for by a difference in the resistance of those 

 attacked by it. The weakest individuals, who are the first to 

 succumb to the infection, also offer the poorest defence, 

 whereas in the later stages the less fit have already been 

 weeded out and the more robust alone remain to be attacked, 

 and these^offer a more stubborn resistance. Such an explana- 

 tion might hold good in the case of a strictly localised 



