246 AGRICULTURAL BACTERIOLOGY 



infected individuals recover. This can not be explained by 

 the greater resistance of the second group over the first, but 

 rather the explanation is to be sought in the diminished 

 virulence of the organism. As has been stated, the causes 

 that induce such changes in nature are not known. 



The first effort to impart immunity by artificial means 

 was by the intentional inoculation of individuals with ma- 

 terial taken from mild cases of smallpox. The mild attack 

 thus induced afforded protection to the individual against 

 the more severe form of the disease. Later it was noted by 

 Jenner, an English surgeon, that those individuals that 

 had acquired cowpox by milking a cow suffering from this 

 trouble were thereby protected against smallpox. Follow- 

 ing this observation, the inoculation of human virus against 

 smallpox was superseded by vaccination with material taken 

 from the pustules of the animal disease, cowpox, or vaccinia, 

 as it is called. 



It is now known that the organism causing cowpox is a 

 modified form of the smallpox virus. In some manner its 

 residence in the body of cattle has so changed its properties 

 that it is no longer able to produce a dangerous form of the 

 disease in man, but it is able to stimulate the tissues to 

 manufacture sufficient anti-bodies to protect the body for 

 a number of years against a natural attack. The vaccine 

 used for inoculation contains the virus of smallpox, the na- 

 ture of which is unknown. 



All vaccines that are used as a protective measure against 

 any contagious disease contain the virus of the disease 

 against which protection is sought. The virus may be viru- 

 lent ; it may be attenuated or weakened ; or it may be dead. 

 The degree of protection afforded by the vaccination pro- 

 cess will depend on the extent to which the organism has 

 been attenuated. Thus the protection afforded by "killed" 

 organisms is not so great as when the weakened organisms 



