FILTERABLE VIRUSES 125 



back even such tiny particles as bacteria. Such filters 

 are usually made of an unglazed, burnt clay. A number 

 of different makes are on the market Berkefeld, 

 Pasteur, Chamberland, Pukall, etc. In all of these it is 

 necessary to either draw the fluid through by suction or 

 force it through by pressure. Figure 40 shows the con- 

 struction of this type of filter. 



Experiments conducted with filters of this type led 

 to the astonishing discovery that the virus of certain 

 infectious diseases was able to pass through. It was 

 impossible to see any living particles in these filtrates 

 even with the highest powers of the microscope, and yet 

 the virus was present, as could be shown by appropriate 

 animal experiments. Very little is known about the 

 nature of these filterable viruses,^ but we have felt it well 

 to mention the fact that they exist. Some of the dis- 

 eases known to be caused by a filterable virus are: 



Measles. 



Hydrophobia (rabies). 



Poliomyelitis. 



Yellow fever. 



Dengue. 



Foot-and-mouth disease of cattle. 



Rinderpest of cattle. - 



Hog cholera. 



So far as hydrophobia is concerned, it appears that 

 some of the finer grained porcelain filters hold back the 

 virus. Certain characteristic bodies found in the brain 

 cells of rabid animals are diagnostic of the disease. 

 They are spoken of as "Negri bodies/' and are held by 

 some observers to be the causative organisms, i. e,, 

 the virus. 



