CHAPTER XXVIII. 
FILTERABLE VIRUSES. RICKETTSIA. DISEASES OF 
UNKNOWN ETIOLOGY. 
Filterable Viritses. Trench Disease. 
Anterior Poliomyelitis. i Rocky Mountain Spotted Fever. 
Lethargic Encephalitis. [ Diseases of Unknown Etiology 
Foot and Mouth Disease. | Measles. 
Contagious Pleuropneumonia of Cattle, i Rabies. 
Bacteriophage. | Trachoma. 
Rickettsia. Smallpox (Variola and Vaccinia). 
Typhus. I Mumps. 
FILTERABLE VIRUSES. ^ 
The viruses of certain diseases of plants, animals and of man are 
fully virulent after they have been passed, suspended in fluid, through 
filters of unglazed porcelain or diatomaceous earth of definite degrees 
of fineness. These filters will not permit the passage of organisms 
as minute as Bacillus melitensis, but Theobald Smith and Wherry^ 
have shown that the bacillus of guinea-pig pneumonia, an actively 
motile bacillus 0.3 to 0.5 micron in diameter and 0.7 micron in length, 
also may pass through such filters unharmed. While filtration in 
general separates smaller from larger bodies, it must always be borne 
in mind that two or even more filterable viruses may pass the pores of 
the filter at the same time. 
The restraining action of filters of unglazed porcelain and diato- 
maceous earth appears to depend in part upon the tortuous passages 
in the walls of the filter than upon the ultimate minuteness of these 
channels. This possibility is suggested by experiments using filters 
of theoretically equal degrees of fineness of material, but of varying 
thickness; it has been shown that bacteria may be forced through 
the thinner walled filter, but not through the thicker. Other factors 
also play a part, however. The amount of pressure positive, or nega- 
tive employed, the temperature and the duration of the operation 
are important. Larger bacteria, B. typhosus for example, will pass 
through filters, provided time enough for their development is given.^ 
The supposition is that the organisms grow around and through 
tortuous passages which eftectually hold the bacteria in the channels 
when pressure is applied. For this reason filtration must not be pro- 
1 See Wolbach (Jour. Med. Res., 1913, 17, 1) for resume of literature to 1913:. King 
(Interstate Med. Jour., 1918, 25, No. 9). Also an excellent review by Rivers (Jour. 
Bacterid., 1927, 14, 217; Am. Jour. Path., 1928, 4, 91). 
2 Jour. Med. Res., 1902, 8, 322. 
3 Smith and Moore: Centralbl. f. Bakteriol., 1892, 12, 628. 
