INFECTION AND IMMUNITY. 171 



always be borne in mind that this classification is not fundamental as is 

 that by which the infectious diseases as a whole are set apart from other 

 diseases, but is closely dependent upon the sanitary conditions under 

 which each case may be placed. Thus tuberculosis, or diphtheria, or 

 pneumonia may be high on the list as readily communicable, if the pa- 

 tient be housed in a crowded tenement with ignorant or careless attend- 

 ance, while if subjected to the intelligent ministry of sanitary science 

 these diseases may be accounted as relatively slightly communicable. ' 



IMMUNITY. 

 NATURE AND FOEMS OF IMMUNITY. 



We have seen that an infectious disease is one incited by the entrance into 

 the body and proliferation there of pathogenic micro-organisms, and that in- 

 fection is the act or process by which such a disease is incited. 



The fact that all animals are not equally susceptible to the ravages of 

 pathogenic micro-organisms, and that in man an individual and often a 

 changing predisposition or invulnerability to the incursions of these or- 

 ganisms exists ; the further observation that one attack of an infectious 

 disease often protects the victim for a longer or shorter time against a 

 recurrence; finally, the fact that recovery is ever possible when once 

 self -multiplying disease-producing germs have obtained a foothold in the 

 body all these facts and observations are of such singular import and 

 interest that, especially of late years, much study has been expended on 

 the nature of the agencies which the body brings into play in establish- 

 ing immunity in the face of microbic invasion, and in coping with the 

 various deleterious factors at work when once a foothold is obtained. 

 The scope of this book does not permit us to enter in detail into this most 

 fascinating and important field. We can give only a brief summary of 

 some of the more important features. 



Immunity is insusceptibility, or capacity for resistance on the part of the 

 body to disease, or in the more limited sense to infection or intoxication or their 

 effects. 



If we recall the ways in which bacteria damage the organism, it will 

 be evident that immunity may be due to the fact that the micro-organ- 

 isms in question simply do not proliferate in the body, failing, even 

 should they gain entrance, to find the necessary conditions. 2 On the 



1 Before the knowledge of pathogenic micro-organisms had become precise, readily 

 communicable diseases were called contagious in a rather loose and ill-defined way, and 

 the unknown excitant was called the contagium. The word contagious is still used, in 

 various senses, to the detriment of science. We can get along well enough without it 

 by the use of the word communicable as above indicated. But if it must still be cher- 

 ished it might be wisely limited to the exanthemata, whose inciting agents are more 

 readily and commonly transmitted through the air from the body of the patient than 

 are those of any of the other infectious maladies. 



2 For a most suggestive and valuable paper on the adaptation of pathogenic bac- 

 teria to different species of animals see Theobald Smith, Philadelphia Medical Journal, 

 May 5th, 1900. 



