CHAPTER III. 



THE VEGETABLE PARASITES. 



For many years scientific men have been endeavouring to 

 discover the nature of the so-called contagion of the acute 

 specific fevers. Now, each one of these fevers runs a more 

 or less definite course, and presents special characteristics of 

 its own by which it is recognised. The poison of each 

 multiplies in a most marvellous degree, and one diseased 

 animal may spread the fever among countless numbers by 

 the agencies of contagion and infection. 



Even so long ago as the Great Plague of London, the 

 belief was expressed that the pestilence was probably due 

 to some living organism, which entered the blood of man 

 and multiplied there and that the virus was capable of 

 passing from him, through the medium of the air or by 

 actual contact, to others. But in those days men had not 

 the means at hand necessary for the discovery of such living 

 parasites. 



That the poison could not be gaseous or liquid was soon 

 obvious, and hence it was regarded as a solid in a state of 

 fine division, and has been proved to be insoluble in fluids. 

 These facts, as well as its remarkable power of multipli- 

 cation, seemed to indicate that the contagion was some 

 living organism; hence the origin of the germ theory of 

 disease. 



In 1838 Bassi and Audouin discovered the fungous nature 



