142 THE HORSE IN HEALTH AND DISEASE 



micro-organisms or viruses. Immunity may be either natural or 

 acquired. 



Natural immunity is an inherited characteristic. It may be 

 an attribute of the species, the race, or the individual. Hog- 

 cholera, a typical infectious disease, will nicely serve to illustrate 

 these three forms of natural immunity. Horses possess a natural 

 immunity common to the species against this disease. If the efforts 

 to produce a race or breed of hogs immune to cholera results suc- 

 cessfully they will have a racial immunity. An example of indi- 

 vidual immunity is often seen in outbreaks of hog-cholera, in which 

 a few hogs will pass through the infection without getting sick, 

 although hundreds die. The individuals in these cases must be 

 above the average of the species in immunity. It seems reason- 

 able to attribute their protection to such factors as "strong consti- 

 tution" or "good condition" rather than to specific bactericidal 

 and other properties of their blood-serum. 



Acquired immunity is the result of some influence acting after 

 birth. This influence may be an attack of the disease, or the injec- 

 tion of preformed immune bodies (antisera). When acquired 

 immunity is established through the activities of the individual's 

 body-cells it is termed active immunity. Such protection is often 

 permanent and lasts for life. An example of acquired active im- 

 munity follows the simultaneous treatment or vaccination of hogs 

 to immunize them against cholera; the disease-producing substance 

 (virus) is actually introduced into the animals to deliberately in- 

 fect them. At the same time a protective immune antiserum is 

 injected to assist the body in overcoming the effects of the disease. 

 In case only the antiserum is injected into the hogs a passive im- 

 munity results, since a preformed immunizing substance is intro- 

 duced instead of one produced by an active process on the part of 

 the animals. Unlike the active type, passive immunity is tempo- 

 rary and of relatively short duration. Depending upon the dis- 

 ease which stimulates the body to produce immunizing substances, 

 or the character of these substances, the immunity may be either 

 antibacterial or antitoxic. 



Susceptibility is the opposite of immunity, but, like immunity, 

 is inherited, at least when particular to a species. The resist- 

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

 and virulent organisms, with the body defences off guard, take 

 advantage of the opportunity to rapidly increase in number and 



