THE PROBLEM OF VIRULENCE 8 



see, now recognized as one of the most important difficulties of sani- 

 tary prophylaxis. In the case of typhoid fever this is particularly 

 true, for it is now well known that a perfectly healthy individual may 

 harbor typhoid bacilli in the gall-bladder for years and constitute, 

 through all this time, a constant focus of danger to the public health. 



The accomplishment of an infection, then, is not determined I 

 merely by the fact that a micro-organism of a pathogenic species! 

 finds lodgment in or upon the body of a susceptible individual, but I 

 it is further necessary that the invading germ shall be capable of | 

 maintaining itself, multiplying and functionating within the new 

 environment. An infection, then, or an infectious disease, is the 

 product of the two factors, invading germ and invaded subject, each 

 factor itself influenced by a number of secondary modifying circum- 

 stances, and both influenced materially by such fortuitous conditions 

 as the number or dose of the infecting bacteria, their path of en- 

 trance into the body, and the environmental conditions under which 

 the struggle is maintained. 



We have in truth, then, a battle of two opposed forces, the result 

 of which is infectious disease. And it is the systematic analysis of 

 these forces in their variable conditions, and the laws which govern 

 them, which constitutes the science of immunity. It is the initial 

 skirmish between the two which determines whether or not a foot- 

 hold shall be gained upon the body of the subject and an infection 

 thus established, and it is the balance between them which decides 

 the eventual outcome of recovery or death. And though it is un- 

 fortunately true that much of the knowledge gained by such studies 

 has yielded no direct therapeutic results, the facts that have been 

 revealed are fundamental to the pathology of infectious disease and 

 as essential to the clinical understanding of these maladies as is the 

 knowledge of the mechanism of the circulation, the chemistry of 

 metabolism, or the structural changes of the tissues to the compre- 

 hension of other pathological conditions. 



And from this point of view the study of infectious diseases can 

 be made an eminently logical one, in that, knowing the criteria 

 which govern the infection of a human being with a given germ, 

 knowing the probable path of entrance, manner of distribution, and 

 biological activities of the micro-organism, and the peculiarities of 

 the mechanism of resistance set in motion in the body by this par- 

 ticular infection, definite clinical deductions can often be made. 



One of the most fundamental facts, immediately apparent on 

 considering the problems of infection, is the phenomenon that among 

 the innumerable varieties of bacteria and protozoa present in nature 

 there is a very limited group which is capable of becoming parasitic 

 upon the body of higher animals, and among these a still smaller 

 proportion which is capable of being "pathogenic' 7 or causing dis- 

 ease. We have used the terms pathogenic and non-pathogenic as 



