DISEASE IN GENERAL 187 



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. Affected animals may never be" decidedly sick, but gradu- 

 ally lose flesh and vigor until they die or become so worthless 

 that they are killed. 



THE CAUSES OF DISEASE 



Etiology is the division of pathology that relates to the causes 

 of disease. It embraces the study of a great variety of external 

 and internal influences that operate to produce disease. Most 

 diseases are caused by external influences, even those that appar- 

 ently originate within the body. For example, faulty body 

 conformation, a hereditary defect, would seem to be of internal 

 origin, but it must have had its external cause in a previous 

 generation as the result of some external influence. 



Disease is produced by predisposing and determining causes. 

 Either one of these may be as extensive in exerting damage as 

 the other. Both include agents that operate from the outside 

 as well as from the inside of the body. 



I. Predisposing Causes. — These are also spoken of as indirect, 

 remote, distant, or accessory causes because they produce a 

 disposition toward disease. They simply prepare the way for 

 easier attack by the determining causes, to be described later, by 

 sensitizing the body. Often they are responsible for the appear- 

 ance of disease in the animals on one farm, while those on a 

 neighboring farm remain healthy. 



The following agents are the main predisposing causes of 

 disease. Without a thorough understanding of them it is certain 

 that one cannot intelligently check or prevent recurrences of 

 disease : 



(1) Climate influences the health of animals and causes disease 

 indirectly by being too warm or too cold. A warm, moist climate 



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