CHAPTER XIV 
IMMUNITY AND PROTECTIVE INOCULATION 
General statement. The application of the principles of immunity 
is an important factor in the control of infectious diseases of animals. 
The prevention of these maladies depends upon two procedures, 
namely, first in keeping the virus of the diseases away from the ani- 
mals and secondly in immunizing animals against them. Successful 
control requires both, for it is not possible in all cases to prevent 
exposure and again a complete immunity cannot be artificially in- | 
duced in all individuals. The practitioner, therefore, should be 
familiar with the principles of immunity and should be ready to apply 
them at any and all times when the checking of an epizodtic disease 
depends upon accelerating the resisting power of the exposed individ- 
uals. For the reasons above stated a short discussion of immunity 
follows. 
Immunity. In a broad sense, immunity is “resistance to disease.” 
The term, however, is usually restricted to the infectious maladies and 
signifies a condition of the individual which enables it to successfully 
defend itself against the invasion of its tissues with infecting micro- 
organisms or to resist the toxic effect of the invading organisms should 
they gain entrance and multiply within the body. It applies to the 
action of all pathogenic bacteria and protozoa. It will be seen that 
immunity is only relative; it is neither permanent nor constant but 
varies with natural and artificial conditions. According to the 
process by which it is established in the individual, immunity is 
recognized as natural or acquired. 
Natural immunity. The term natural immunity is applied to that 
condition which enables animals to resist the natural invasion of 
infecting microorganisms that attack other varieties or species of 
animals. It is a condition inherent in the nature of the individual, 
born with it and transmitted to its offspring. 
There are a number of very striking examples of natural immunity. 
The Algerian race of sheep are immune to natural infection of anthrax, 
whereas other sheep are very susceptible to it. The equine species is 
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