200 PRINCIPLES OF VETERINARY SCIENCE 



properly extracted and prepared, will prevent the same diseases 

 from developing in other animals that are liable to them. When 

 such serum is injected into another animal, a passive or temporary 

 immunity is produced quite rapidly with no danger and little 

 discomfort. The passively immunized animal is simply the 

 recipient of the protective substances or antibodies formed in 

 the body of other animals and transferred to it. An example of 

 this form of immunity is seen in hogs after they have been given 

 the serum treatment to protect them from hog cholera. 



It is a matter of common knowledge that very young animals 

 show a definite resistance to diseases that they later get. This 

 is explained by the fact that the mother's milk contains specific 

 protective principles. The immunity, being but passive, gradu- 

 ally disappears after weaning and then the animals become readily 

 infectible. Not uncommonly disease germs, after lying dormant 

 for a long time, suddenly take on new life when the disease is 

 said to "light up." This is due largely to reduced resistance of 

 the animal. In other instances, after a variable course, diseases 

 "die out," due to increased resistance on the part of the infected 

 animal. 



Susceptibility is the opposite of immunity. It is shown as 

 sensitiveness or predisposition to disease. Like immunity, it is 

 inherited, at least when particular to a species. The resistance 

 of the animal is lowered, the normal equilibrium is disturbed, 

 and virulent organisms with the body defenses off guard take 

 advantage of the opportunity rapidly to increase in number and 

 to produce disease. It is not uncommon to find certain animals 

 in a herd highly susceptible or predisposed to a specific disease, 

 while the rest of the herd shows great resistance. Experience 

 goes to show that individual susceptibility is often due to a 

 "weak constitution," "poor condition," temporary physical 

 depression, fatigue, exposure to cold, poor hygiene, or poisonous 

 gases. Many of the terrible epizootics or "animal plagues" 

 of ancient times were probably caused by the introduction of 

 some hew germ against which the animals had no natural im- 

 munity and toward which they were particularly susceptible. 



Constitution is an important factor in determining the resistive 

 influences possessed by the body against disease. An animal is 

 said to have a "strong constitution" when his resistive powers are 

 highly developed and he is healthy and vigorous; and a "weak 



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