150 MICROBES AND TOXINS 



a series of different forms (cf. malaria) which appear and 

 disappear in the blood. These forms are unknown and must 

 'be extremely minute. 



From the immunity point of view also the protozoal diseases 

 present characters quite different from the bacterial infections. 



The Viruses Capable of Passing Filters. 



The bacteria which we study under the microscope are 

 unequal in size. The Bacillus Biitschlii we mentioned in 

 connection with the nucleus of bacteria is a colossus in com- 

 parison with the bacilli of fowl-cholera, with the Micrococcus 

 parvulus of Veillon, or even with the little bacillus found 

 in influenza by Pfeiffer. There probably exist bacteria still 

 smaller. Our best microscopes do not allow us to dis- 

 tinguish a particle whose thickness is less than o'l ju. The 

 bacteria smaller than ot /* are therefore invisible under the 

 microscope ; they are ultra-microscopic. Since there are many 

 diseases in which the microbe remains unknown we are 

 tempted to ascribe to them ultra-microscopic microbic agents. 

 Already in 1884 Pasteur said that the virus of rabies was too 

 small for us to be able to see it. 



The study of these extremely small microbes only commenced 

 in 1898 with an experiment by Lbfflerand Frosch on the virus 

 of foot-and-mouth disease, which no one has yet made visible. 

 The serous fluid from an ulcer (in which no microbe can be seen) 

 is diluted with water and filtered through a porcelain bougie 

 (similar to those of the Chamberland filters) ; there results a 

 perfectly clear fluid free from visible microbes which is capable 

 of transmitting the disease to a fresh animal ; this is the first 

 example of a virus passing through filters, or, as it is commonly 

 called, a filtrable virus. 



Since 1898, the existence of filtrable viruses has been proved 

 by experiments in about twenty diseases, the chief of which 

 are foot-and-mouth disease, pleuropneumonia of cattle (rinder- 

 pest), yellow fever, swine-plague, cattle-plague, small-pox, and 

 rabies. 



