WHAT IS A DISEASE? 415 



lying cause. Even the same rash, e.g. psoriasis, which, when 

 uncorrelated with other disorders is called a disease, becomes, 

 when correlated with other disorders, such as those of gout, a 

 symptom. Caries of the teeth, seeing that it cannot be corre- 

 lated with any other disorder, is clearly a disease. It is the 

 whole of the disorder that can be attributed to a single intra- 

 corporeal cause. 



Another difficulty in applying the definition may be felt 

 when the correlating cause of the disorder is widespread, but 

 concentrates its attack mainly upon a single organ. Few 

 diseases are more distinctively diseases, or are better entitled 

 to be called a disease, than Addison's disease ; yet in Addison's 

 disease the correlating cause is usually tuberculous disease of 

 the supra-renals, a local manifestation of a tuberculosis which 

 may exist in other parts of the body also. In this case, the 

 sum of the correlated disorders includes more disorders than 

 are included in the concept of Addison's disease ; and Addison's 

 disease stands in the same relation to tuberculosis as endocar- 

 ditis to acute rheumatism. It is a sub-disease of tuberculosis. 

 It is a local focus, carrying its own symptoms and consequences, 

 of a general infection. Gumma of the brain or of the liver 

 stands in the same relation to syphilis. All these are sub- 

 diseases ; but there are practical reasons which render it 

 expedient to put them in a different position. Addison's 

 disease is still Addison's disease, whether the destruction of 

 the supra-renals is due to tuberculosis or to cancer. Tumour 

 of the brain or of the liver has its own characteristic symptoms, 

 which with the tumour make up the disease, whether the 

 tumour is syphilitic, or tubercular, or gliomatous, or cancerous. 

 It is therefore practically convenient to place the chronic 

 infections, such as tuberculosis, syphilis, cancer, hydatids, 

 and so forth, in a separate class, and to regard each of them 

 as capable of producing various- diseases according as their 

 attack is focussed in this organ or in that, reserving the name 

 of ' a disease ' for those clinical aggregates that result from 

 the localisation of an infection in a particular organ. 



The presence of certain other correlatives besides disorders 

 of function is sometimes allowed to enter into the concept of 

 a disease. Instances of such correlatives are the time of life 

 at which the disorder occurs, and the extra-corporeal cause to 

 which it is due. By taking into the correlate the time of life 



