CAUSATION OF DISEASE 305 



site in 1880.^ Physicians ascribed the plague which visited 

 Southern France in 1721 to the same cause, and many even 

 went so far as to attribute all disease to animalcules, which 

 brought the theory into ridicule. Nevertheless the "con- 

 tagium vivum" theory survived, and even Linnaeus, in his 

 Systema Nature (1753-1756), recognized it by placing the 

 organisms of Leeuwenhoek, the contagia of diseases and the 

 causes of putrefaction and fermentation in one class called 

 "Chaos." 



Plenciz, a prominent physician and professor in the Vienna 

 Medical School, published in 1762 a work in which he gave 

 strong arguments for the "living cause" theory for trans- 

 missible diseases. He taught that the agent is evidently 

 transmitted through the air and that there is a certain 

 period of incubation pointing to a multiplication within the 

 body. He also believed that there was a specific agent 

 for each disease. His writings attracted little attention at 

 the time and the "contagium vivum" theory seems to have 

 been almost lost sight of for more than fifty years. Indeed, 

 Oznam, in 1820, said it was no use to waste time in refuting 

 hypotheses as to the animal nature of contagium. 



Isolated observers were, however, keeping the idea alive, 

 each in his own locality. In 1787, Wollstein, of Vienna, 

 showed that the pus from horses with glanders could infect 

 other horses if inoculated into the skin. Abilgaard, of 

 Copenhagen, made similar experiments at about the same 

 time. In 1797, Eric Viborg, a pupil of Abilgaard's, published 

 experiments in which he showed the infectious nature not 

 only of the pus but also of the nasal discharges, saliva 

 urine, etc., of gland ered horses. Jenner in 1795-1798 intro- 

 duced vaccination as a method of preventing smallpox. This 

 epoch-making discovery attracted world-wide attention and 

 led to the overcoming of this scourge which had devastated 

 Europe for centuries, but contributed little or nothing to 

 the question of the causation of disease. Prevost's discovery 

 of the cause of grain rust {Puccinia graminis) in 1807 was 



^ Sir H. A. Blake has called attention to the fact that the "mosquito 

 theory" of malaria is mentioned in a Sanscrit manuscript of about the 

 sixth century a, d. 

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