Transmissible Diseases. 9$ 



of a second host may occur the parasite must escape from 

 the body of the first in some manner. The parasitic bac- 

 teria can not, as a rule, readily multiply outside the body 

 of the proper host. The opportunity for direct trans- 

 mission from the sick animal to another healthy one is 

 always so great that the infection is apt to be spread in 

 this immediate way. Even where direct transmission 

 from the one animal to another does not occur, the escape 

 of the causal parasite into the outside environment per- 

 mits of indirect infection through accidental contamina- 

 tion of water, food and other objects. 



Each disease is caused by a specific organism. No 

 other organism can cause the disease and without the 

 proper organism the disease is impossible. Each case of 

 the disease thus demands a previous case. Sometimes 

 the relation between the two is easily established, often it 

 can not be traced. In order to prevent or treat any 

 transmissible disease, it is of advantage to know the cause 

 of the disease, the nature of the causal organism, the way 

 in which it enters the body of the animal, the portions of 

 the body in which it grows, the changes it produces, and 

 lastly, the ways in which it leaves the body of the affected 

 animal. Unless these things are known, many things 

 will be left undone that are necessary to prevent the 

 spread of the disease. Often many things are done 

 which have no importance in preventing the spread of 

 the trouble. 



Portals of entrance of bacteria. The bacteria enter 

 the body of the animal in a number of ways. The skin 

 acts as a protective layer to the body proper, and as long 

 as it is intact prevents the entrance of bacteria into the 

 body. As soon as it is cut, punctured or bruised, oppor- 

 tunity is offered for the entrance of bacteria. These are 



