DISEASE IN GENERAL 137 



In many treatises diseases are classified, on an anatomic basis, 

 as respiratory, nervous, urogenital, etc., in a manner similar to 

 that followed in classifying the different systems of body organs. 

 A simple grouping of diseases according to their manner of occur- 

 rence follows: (1) Sporadic disease — one which occurs in isolated 

 cases or at rare intervals, for example, glanders. (2) Epizootic 

 disease — one which occurs in many animals or over a large area 

 at the same time, like influenza. (3) Enzootic disease — one which 

 exists among small numbers of animals or within a restricted area, 

 like heaves. 



It is customary to refer to diseases according to their duration, 

 as: (1) Acute disease — one which runs a rapid course of a few days, 

 and comes to termination within a time that experience has shown 

 to be fairly constant for that disease, like azoturia. (2) Subacute 

 disease — one which runs a slower course than an acute disease, and 

 lasts for two or three weeks, like glanders in the mule. (3) 

 Chronic disease — one which runs a prolonged course of from four 

 weeks to an indefinite period of months, or years, or decades, like 

 tuberculosis. In chronic diseases there is no regularity of events 

 in the symptoms which follow. We cannot say when the disease 

 began or when it will end. 



THE CAUSES OF DISEASE 



It is evident that there must be many different causes of dis- 

 ease. In a broad sense, all diseases are caused by influences which 

 originate outside the body. For example, faulty body conforma- 

 tion, a hereditary defect, would seem to be of internal origin, 

 but this alteration in conformation must have had its external 

 cause in some previous generation as the result of external influ- 

 ence. 



The causes of disease may be conveniently grouped as pre- 

 disposing and determining causes. One of these may be as exten- 

 sive in exerting damage as the other. 



(A) Predisposing causes of disease are remote or distant causes 

 which produce a disposition toward disease; that is, prepare the 

 patient for easier attack by the determining causes. For the most 

 part, the predisposing causes exert their influence from the out- 

 side. It should not be understood, however, that all external in- 

 fluences are predisposing causes, as reference to those classified 



