4 INTRODUCTION PATHOGENICITY 



As a matter of high theory, therefore, there is no fundamental 

 distinction between pathogenic and non-pathogenic bacteria, and 

 we can imagine circumstances in which the tissues are vulnerable 

 to attack by almost any microbic species. Practically, however, 

 we shall consider an organism as pathogenic when the immunity 

 of the animal which it attacks is not so perfectly developed that 

 its presence in the tissues is but transient and unaccompanied by 

 any noticeable ill-effects, but in which there is a balanced contest 

 of longer or shorter duration between the injurious powers of the 

 microbe and the defensive mechanism of the host, accompanied 

 by more or less injury to the tissues and disturbances of the 

 physiological economy of the latter, and resulting either in the 

 death of the invader or of the patient. All grades occur. In 

 most staphylococcic infections the chances are enormously on 

 the side of the host, and the immunity is sufficiently high to 

 localize the process before it has gone far. In typhoid fever the 

 natural immunity and the pathogenic power of the organisms are 

 more nicely matched ; the contest between them is of long duration 

 and doubtful issue. And in some forms of human disease, but 

 more especially in artificial infections of animals with highly 

 virulent cultures, the power of immunity seems almost nothing, 

 the bacterium growing apparently unchecked and death occurring 

 within a few hours. We say that these organisms have different 

 degrees of pathogenicity, but it would be equally correct to say 

 that there are different degrees of resistance against them, since an 

 organism that is highly virulent towards one animal species may 

 be quite harmless to another, so that pathogenicity is not an 

 inherent property of certain bacteria. 



Thus far we have considered the resistance of the host as if it 

 were fixed and definite, but this is not the case. It has been 

 known from time immemorial that certain diseases especially 

 those due to infection are followed by a greater or smaller degree 

 of immunity, so that a second attack is unlikely at any rate, for 

 some time. Smallpox, scarlet fever, and measles are amongst the 

 most striking examples, and in them the protection given by the 

 disease is in most instances absolute and lifelong. This is known 

 as acquired immunity, and we shall enunciate it as a law that all 

 recovery from infective disease is due to, and followed by, some 

 degree of acquired immunity, though this may be slight, transient, 

 and perhaps local. 



Take, for example, a case of pneumonia, a disease which may 



