CHAPTER XXI 

 SOME COMMUNICABLE DISEASES 1 



Communicable diseases are also known as infectious, con- 

 tagious, and preventable. They are infectious from the fact 

 that each is produced by the entrance into the body of a specific 

 or single disease-producing or infectious microorganism to which 

 the body reacts by showing characteristic symptoms and lesions. 

 They are contagious because they pass to other animals of the 

 same or other susceptible species by direct contact. They are 

 preventable because it is possible to prevent their spread by stop- 

 ping the transmission of the microorganisms to other animals. 



As a rule, the bacteria of these diseases do not produce typical 

 wound infections following introduction into the tissues. Al- 

 though our present methods of study have failed to discover the 

 germ of a few communicable diseases, it is certain that some 

 form of germ life is responsible for each. The term virus is 

 employed to designate the cause of those infectious diseases of 

 which the organism is so small that it cannot be seen with the 

 microscope and will pass through a very fine filter. 



As with the non-communicable diseases, secondary infections 

 are often implanted upon or associated with the primary causes 

 of communicable maladies. Most of the secondary infections 

 are caused by pus-producing organisms. In many cases these 

 germs gain admission to the blood-stream and are carried to all 

 parts of the body. When they reach an organ having a weak 

 resistance, they lodge, immediately start to multiply, and cause 

 serious interference with its functions, complicating the original 

 disturbance, rendering diagnosis more difficult, and hastening 

 death. 



The time which elapses between the exposure of the animal to 

 infection and the appearance of visible symptoms of the disease is 

 called the period of incubation. It varies from a few days in very 

 acute diseases to several weeks in chronic affections. Usually 



1 See appendix for outline for the study of the communicable diseases. 



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