96 Agricultural Bacteriology. 



subject to the disease, as with tuberculosis which natur- 

 ally occurs in man, cattle, hogs, fowls, and many other 

 animals. 



Period of incubation of transmissible diseases. A 

 period always elapses between the time of the entrance 

 of the bacteria into the body and the first signs of illness. 

 This is called the " period of incubation" of the disease. 

 It may be but a few days in length, as in the case of 

 anthrax, or from several weeks to months, as with tuber- 

 culosis. 



Lesions of the disease. The bacteria produce in the 

 body of the affected animal more or less characteristic 

 symptoms and changes. By these alone, one is often 

 able to determine the disease present. The changes in 

 the different organs of the body from their normal condi- 

 tion, due to the action of the disease-producing organ- 

 ism, are known as the ' ' lesions. ' ' A post-mortem exami- 

 nation of any animal that has died of a suspected trans- 

 missible disease should always be made so as to render- 

 more certain a correct diagnosis. Precautionary meas- 

 ures which will be described later should, however, be 

 taken in making such examinations, in order to prevent 

 further trouble. 



The various transmissible diseases differ greatly in the 

 rapidity with which they progress in the animal. An- 

 thrax usually kills the animal in a few days, while with 

 tuberculosis an animal may live for years. 



Necessity of diagnosis of transmissible disease. With 

 those diseases caused by some form of bodily disturbance, 

 it is of small importance, except for determining the kind 

 of treatment to apply, whether a correct diagnosis of the 

 disease is made or not, since the disease can not spread to 



