THE PROBLEM OF VIRULENCE 23 



are alike. And cases of this disease in general differ as a class from 

 the maladies caused by, let us say, the group of clinical conditions 

 resulting from anthrax infection, where entrance is through the 

 skin, and generalized infection of the blood ensues without definite 

 or regular localization in any given organ. Again, a localized 

 staphylococcus abscess will differ materially from an equally local- 

 ized focus of tuberculosis, because the chemical constituents of these 

 bacteria respectively call forth each a characteristic response on the 

 part of the defensive mechanism. 



Such specificity of the various micro-organisms may of course 

 be due partly to their mode of attack and distribution, and partly, 

 as we shall see, to the pharmacological action of the poisonous prod- 

 ucts given out by them. 



That both factors contribute seems beyond doubt; but recent 

 work, especially that of Friedberger, which is fully discussed in 

 another place (see p. 415), seems to show that clinical differences 

 Depend much less than was formerly supposed upon specificity of the 

 intracellular poisons, and much more upon distribution and localized 

 accumulation of the germs, conditions which are determined rather 

 by the mode and extent of invasion than by chemical differences of 

 poison production. This problem, rather difficult to discuss on the 

 limited basis of the facts so far outlined, will become clearer as we 

 proceed, but we need only refer at present to the essential clinical 

 uniformity of the various forms of septicemia, where organisms 

 freely circulate in the blood with often a focus of distribution on a 

 heart valve conditions in which it is rarely possible to determine 

 the species of the responsible germ except by blood culture. Or, 

 again, as Friedberger 43 points out, there is great similarity between 

 the ordinary pneumococcus pneumonia and that caused by the Fried- 

 lander bacillus. In both cases the distribution and mode of attack of 

 the bacteria are essentially the same, though the micro-organisms' 

 themselves are biologically very dissimilar. 



One and the same micro-organism, on the other hand, may cause 

 entirely different clinical conditions, and here the type of infection 

 depends purely on the degree of invasion possible in the given case 

 that is, the balance between virulence and resistance. A germ may 

 enter the body and cause an inflammatory reaction at the point of 

 entrance, the process remaining purely localized. In such cases the 

 defensive forces have been so efficient, the invasive properties of the 

 germ so relatively weak, that progression beyond the point of en- 

 trance is prevented and the resultant disease takes the form merely 

 of a localized abscess. This is the case when a healthy individual 

 is infected with an attenuated organism or by one whose species' 

 characteristics do not include a powerful invasive property. Thus 

 streptococci, if entering the tissues of a normal subject in small 

 43 Friedberger. Deutsche med. Woch., No. 11, 1911. 



