PARASITES 429 



over a hundred years was widely practised until it was pro- 

 hibited by an act of Parliament in 1840. The danger of this 

 practice lies mainly in the fact that persons artificially infected 

 in this way become new centers for the spreading of the 

 disease in the virulent form. 



In 1796, Edward Jenner, a country physician, in England, 

 observed that persons who had been affected with cow-pox, 

 a mild eruptious disease of cattle, were very unlikely to con- 

 tract smallpox even when repeatedly exposed to it. This 

 led him to recommend, and to practise on his patrons the use 

 of the virus of cow-pox instead of that of smallpox as a means 

 of producing immunity to the latter disease. His method 

 proved to be a remarkable success and soon came into general 

 use. At present, the vaccine is produced by inoculating a 

 healthy calf with the cow-pox VIRUS and then after about five 

 days, the virus of the pustules, which develop near the 

 place of inoculation, is removed, mixed with glycerine and 

 kept until it is proved to be free from bacteria and other or- 

 ganisms, and then it is ready for use in vaccination. 



496. Cause of Smallpox not Known. You will note that 

 this method of vaccinating against smallpox was discovered 

 more or less accidentally long before the germ theory of dis- 

 ease came to be generally accepted. Consequently, at the time 

 of its discovery, no one had any idea of the nature of the proc- 

 ess by which the immunity is brought about. We have 

 not even yet been able to discover the organism which is 

 the cause of this disease, and our only reason for believing 

 that it is caused by some microorganism, is its general re- 

 semblance to other diseases that are known to be thus caused. 

 Since we do not know the organism, we can judge of the nature 

 of the immunizing process only by comparison with other 

 better known diseases. From this point of view, it is the 

 opinion of most students of the subject, that the organism 

 which causes cow-pox is the same as the one which causes 

 smallpox and that in passing through the body of the calf, 

 the organism becomes attenuated, or weakened, so that it 



