THE PATHOGENIC PROTOZOA 153 



the time it would be possible to make microbes pass which are 

 not ordinarily " filtrable," but this would not be really a filtration 

 but a culture propagating itself by extension from one side of the 

 filter to the other. This is what happens in the filters of water 

 supplies which are badly kept. Bacteriologically speaking, they 

 are no longer niters at all ; instead they are continually infecting 

 the drinking water with the microbes which they are supposed 

 to be keeping out. 



Filtration can show that some quite minute microbe exists 

 in a given infection : it gives no information as to its nature. 

 Fortunately, the labours of Jenner and Pasteur have proved that 

 it is possible to study a virus without seeing it. It can be 

 purified (precisely by filters as it happens), inoculated, and its 

 resistance to physical and chemical agents (heat, antiseptics, 

 etc.) determined, as well as the conditions of preservation and 

 attenuation. All the viruses enumerated at the head of the 

 chapter have been treated in this way and processes of immun- 

 ization have been discovered against certain of these viruses 

 which are still unknown, a paradox which has become familiar 

 to us through Jennerian vaccination and the.antirabic treatment 

 of Pasteur. 



To say that a virus is filtrable is to give it an external rough 

 definition ; there are undoubtedly in this group very different 

 microbes ; some may be bacteria, others protozoa. Borrel has 

 described a protozoon which passes the rough filters. In the 

 life cycle of the Hamamceba Ziemanni of the little owl, 

 Schaudinn has described motile forms smaller than the 

 microbe of peripneumonia ; it is admitted that even the most 

 visible protozoon may have ultra-microscopic stages. 



Several groups may henceforth be distinguished among the 

 diseases due to filtrable viruses : 



i. Pleuropneumonia of cattle : in this the microbe has been 

 filtered, cultivated, and finally seen. It would seem that it 

 ought to be easy to describe it. At first it was said to be a 

 bacterium, extremely fine cocci namely, which it was possible 

 to see under the ordinary microscope, not singly, but in 

 amorphous masses. Recently, Bordet has described by means 



