CAUSE OF VARIATION 15 
should be done to prevent the virus from spreading from the animals 
in one herd to those in another, or from one individual to another 
in the same herd. The extermination of the virus, or its restriction 
to known places, is dependent upon a knowledge of the ability of the 
specific microérganism to resist destructive influences or to be carried 
from one place to another. It also involves a knowledge of the tissue 
changes and the protective forces of the individuals invaded. These 
determine under what conditions the virus can escape and when 
infected individuals are immediately dangerous. The specific morbid 
anatomy of the infectious diseases is essential for an understanding 
of this phase of the subject. 
Immunization. Tmmunization of animals against either sporadic 
or epizodtic infectious diseases has been found to be practicable 
with certain diseases such as “‘black leg,” hog cholera and a few others. 
This method is valuable when it is possible to establish an artificial 
immunity. The method of vaccination, however, cannot be em- 
ployed in the control of infectious diseases except with those affections 
where efficient immunizing methods are known. The danger of 
the method is, that where it is employed precautions against infection 
are usually minimized and consequently if the protection is not 
complete it gives a false security. To successfully employ immuniz- 
ing processes for the control of infectious diseases it is essential to 
have a thorough knowledge of the principles of immunity, its varie- 
ties and methods for its production. 
Cause for variations in the course of an infectious disease. It 
is recognized that there is much variation in the course of infectious 
diseases in different epizodtics and often marked individual variations 
occur in the same outbreak. In explaining this interesting phenome- 
non, it is important to take into account the question of individual 
résistance or immunity,—partial or more complete. It was found 
in case of certain diseases that when an individual was partially 
immunized and then infected the lesions were very much modi- 
fied. The teachings of a specific etiology point to this phenomenon 
as a result of certain biological or vital differences existing either in 
the invading organism, or in the host, or possibly in both. The results 
of the investigations already made along this line suggest as a prob- 
able explanation, that the course of the disease varies on the one side 
with the immunity possessed by or resistance of the host and on the 
