890 DISEASES CAUSED BY FILTRABLE VIRUS 



organisms like spirochaetes it might be quite possible with sufficient 

 suction and niters of sufficient size to mechanically pull through 

 organisms which, by their actual normal dimensions, would be re- 

 garded as too large to be classified with filtrable organisms merely 

 in regard to size. The Berkefeld filters of va-rious grades will differ 

 materially in the sizes of the particles they will let through, and 

 filters that are new will act differently from filters that have been 

 used; and in this regard will vary to some extent, according to 

 the nature of the substances that they have been used for at previous 

 filtrations. Also, in a comparative experiment made by us in con- 

 nection with syphilis some years ago, it was found that new filters 

 of presumably the same manufacturer's grade, differed somewhat 

 in regard to the substances they allowed to pass through, and all 

 experienced investigators will always control their filtration experi- 

 ments by adding to the substance, the filtrability of which they wish 

 to test, bacteria of a small size (like influenza bacillus or B. pyo- 

 cyaneus), so that they may know in that particular filtration the 

 filter used was impermeable to the small bacteria. Filtrability, then, 

 is a matter of gradation and not a sharp wall which necessarily 

 separates one kingdom of microorganisms from another. 



The following tabulation of the diseases presumably caused by 

 filtrable virus was made by Wolbach 1 some time ago and is taken 

 from his article by us in its entirety with one or two modifications 

 made necessary by recent developments: 



SMALLPOX 



Smallpox or variola is one of the most virulent of infectious 

 diseases. Throughout history it has been a severe scourge of man- 

 kind, prevailing in China and other Eastern countries many centuries 

 before Christ and sweeping through medieval Europe, especially at 

 the time of the Crusades, in a series of severe epidemics. All races 

 of men are susceptible and no age from childhood to senility is 

 exempt. In modern times the disease is endemic in most uncivilized 

 countries, especially those of the East, and occurs sporadically in 

 all parts of the globe. Owing to rigid enforcement of vaccination 

 and of quarantine laws, however, the disease has been practically 

 eradicated from civilized countries. 



1 Wolbach, Jour, of Med. Ees., xxvii, 1912. 



