TWENTY-FIVE YEARS OF BACTERIOLOGY 137 



would induce the disease on inoculation, but that the same 

 series of events followed the dilution and inoculation of 

 the vesicular contents of the experimental variety of dis- 

 ease through an indefinite series. Obviously the filtrate 

 contained a living element which came to be called a 

 virus, just as is the small-pox germ, for in neither instance, 

 and notwithstanding laborious endeavors, has the living 

 organism itself ever been seen under the microscope. 



We now recognize a class of microbes or viruses which 

 are so minute as to be regarded as ultramicroscopic, and 

 yet so active as to be capable of setting up disease in ani- 

 mals and man. The precise limits of the class have yet 

 to be defined. When we consider that there remain still 

 to be detected the microbic incitants of some of the most 

 contagious as well as common of diseases, our minds read- 

 ily seize hold of the possibility of their being of this na- 

 ture. Thus the microbes responsible for such contagious 

 maladies as measles, scarlet fever, and chicken-pox, and 

 those inducing small-pox and rabies are not known, and 

 not a little obscurity still surrounds the etiology, as we 

 say, or immediate origin of epidemic influenza. 



Inasmuch as the filterable microorganisms or viruses, 

 or filter passers as the British prefer to call them, are 

 known alone through their disease-producing propensi- 

 ties, no one can say whether, as is true of the bacteria, 

 innumerable kinds exist in nature, among which relatively 

 a small number has acquired parasitic or pathogenic quali- 

 ties. Of the less than a dozen diseases known or on good 

 grounds considered to be induced by filterable micro- 

 organisms, two attack human beings, namely poliomye- 

 litis or infantile paralysis, and trench fever ; and a third, 

 yellow fever, which until very recently was believed to be- 

 long also in this category, has now been relegated to 

 another class, with respect to which special devices suffice 

 to bring into view its microbic incitant. 



There exists, therefore, a degree of uncertainty in this 

 field of research for which allowance must be made, since 



