186 PRINCIPLES OF VETERINARY SCIENCE 



not able scientifically to interpret the complex picture presented 

 by disease, or to understand the why and wherefore of the process 

 from inception to recovery, for this calls for the training and 

 experience of a professionally educated man. 



It is not the intention of the author to discourage the layman 

 by these remarks, but to show him how absurd it is to guess at 

 changes due to disease that even the most thorough investiga- 

 tion has failed to make clear. . 



Classification of disease on a satisfactory basis has been 

 possible only within the last thirty years, or since the study 

 of bacteriology has revealed the causes of many heretofore 

 obscure diseases. According to cause, diseases are generally 

 recognized as belonging to either one or the other of the following 

 classes: (1) Non -infectious disease — one which is caused by 

 something other than living germs; for example, the disease 

 known .as heaves, in which structural changes are the cause of 

 the disorder. (2) Infectious disease — one which is produced 

 as a result of the successful invasion of the body by living germs; 

 for example, tuberculosis, which is caused by the pathogenic 

 microorganism known as the tubercle bacillus. These are com- 

 monly known as germ diseases. 



Another classification is based upon the nature of the disease 

 from the standpoint of transmission and control as follows: (1) 

 Non-communicable disease — one which is not transmissible by 

 direct contact from one animal to another, like colic. (2) Com- 

 municable disease — one that can be passed on to other animals 

 by direct contact. It is evident that all communicable diseases 

 must be infectious, but that all infectious diseases need not be 

 contagious. 



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 re- 

 stricted area, like heaves. 



It is customary to refer to diseases according to their course and 



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