VACCINATION. IMMUNIZATION. INOCULATION 227 



Practical experience having taught that an acquired immunity 

 often existed, attempts were early made, by intentionally intro- 

 ducing infection, to produce an artificial protection against a 

 subsequent infection. This variety of insusceptibility is called 

 artificial immunity and the process of artificial infection is termed 

 vaccination. The first vaccination for the purpose of producing 

 an artificial immunity was Jenner's vaccination of man with cow 

 pox (1796).' In veterinary medicine, vaccinations of this char- 

 acter were early undertaken (sheep pox, lung plague, rinderpest, 

 foot-and-mouth disease). In more recent times, great scientific 

 and practical interest has been manifested in the production by 

 vaccination of artificial immunity against anthrax and black leg, 

 rabies, swine erysipelas, swine plague, tuberculosis, calf cholera, 

 chicken cholera, contagious pneumonia of horses, and tetanus. 

 Vaccination has become a valuable method of combating the infec- 

 tious diseases and is a species of internal disinfectant and anti- 

 dotal treatment. 



Causes of Immunity. — ^The explanation of the establishment 

 of immunity has always been one of the most difficult therapeutical 

 problems. Only in recent times has light been thrown on the 

 intricate question by histological, bacteriological, and- chemical 

 inquiries. But a conclusive investigation has not been made, nor 

 has an entirely satisfactory explanation of the causes and nature 

 of immunity been discovered. 



The best-known theories at this time are the serum theory of 

 von Behring and the phagocytic theory of Metschnikoff. The 

 first has his supporters chiefly in Germany, the latter in France. 

 The truth may indeed lie between the two; namely, that the serum 

 and the phagocytes both act as protectives of the body against 

 infectious materials. 



' Jenner's celebrated paper, which appeared in London in 1798, bore the 

 title: "An Inquiry into the Causes and Effects of the Variolae Vaccinae, a 

 Disease Discovered in Some of the Western Countiesof England, Particularly 

 Gloucestershire, and Known by the Name of Cowpox." Jenner's discovery 

 has become the greatest therapeutic fact of all times. 



