1919] on A Filter-passing Virus in Certain Diseases 553 



the filtration, so that an organism has sufficient time to actually grow- 

 through the filter. In the hands of careful and experienced workers, 

 and with frequent suitable controls and tests, the use of such filters 

 is most valuable, and in fact indispensable to such work as at present 

 concerns us. These filters were used in the first instance, and are 

 still used, for the purpose of separating organisms from toxins, and 

 the filtrates obtained were sterile, toxic, but not infective, in their 

 action. 



In the year 1898 a remarkable discovery was made by Loffler 

 and Frosch. These observers were studying foot-and-mouth disease, 

 a contagious malady affecting both man and cattle, and were filtering 

 lymph from infected animals on the usukl lines with the object of 

 obtaining a toxic but germ-free lymph. They found, however, that 

 a minute quantity of the filtrate, i.e. 0'02 c.c.,-was still capable of 

 producing the infection, although they were unable either to see or to 

 culture any organism from the filtrate. This was the real origin of 

 the belief in filtrable or filter-passing viruses, i.e. living organisms so 

 small that they would pass through the minute pores of a porcelain 

 filter that held back all the hitherto known varieties of micro- 

 organisms. It was also supposed that such forms might be ultra- 

 microscopic, but doubt was soon thrown on this latter hypothesis, as 

 Roux and Xocard in the same year, 1898, succeeded by a special 

 method in showing that the filter-passing organism of pleuro- 

 pneumonia was visible as a minute dot under a magnification of 

 2000 diameters. It was thus clear that certain organisms were 

 entitled to be called filtrable or filter-passing, but no confirmation 

 of the theory of their being ultra-microscopic was obtained. 



From this time onward additions to knowledge were made, so that 

 at the present time a considerable number of human and animal 

 diseases are regarded as due to filtrable viruses, but it will suffice 

 here to draw attention to two discoveries of importance. In 1903 

 Dorset and McBryde produced evidence to show that swine fever, 

 the aetiology of which was very obscure, was really due to a filtrable 

 virus, and that the bacillus to which it had been attributed before 

 was really an associated organism. This observation was of very 

 considerable importance, in that it suggested the possibility that 

 some diseases where there was much confusion and discordance w^ere 

 really due to the action of two viruses, a filtrable virus as the primary 

 and essential cause, and then a second constantly associated organism 

 of a different type, e.g. a bacillus. This is an important result that 

 may have applications to some human diseases of doubtful causation. 

 The other discovery was also an extremely important one, having a 

 very direct bearing on the matter before us, i.e. the elaboration of a 

 method by which a virus capable of passing through certain filters 

 was successfully grown and rendered clearly visible by staining. This 

 was a great step in advance, as it removed some of the mystery that 

 up to this time surrounded the filtrable viruses, and showed that in , 



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