FILTERABLE VIRUSES 



541 



1. Vaccinia. 



2. Variola. 



3. Scarlet fever. 



4. Hydrophobia. 



The Cytooikon Group, 



The Cytooikon group includes : — ■ 



I Lyozoon atrophicans Leber and Prowazek. 

 2. Trachoma bodies. 



The former group have been described above, and the latter group 

 will be dealt with in the chapter dealing with the diseases of the 

 special senses. 



The Filterable Viruses. 



By the term ' filterable virus ' is meant micro-organisms so small 

 that they will pass through the pores of filters which are too small to 

 allow the passage of bacteria. The term ' ultramicroscopic ' is 

 also used for filterable viruses, but is not free from objection. 

 ^.History. — In 1892 Ivfanowski demonstrated the filterability of the 

 m^osaic disease of the tobacco plant, and in 1898 Loffler and Frosch 

 discovered that the virus of foot and mouth disease would pass 

 through the pores of the finest porcelain filter. Shortly after- 

 wards Beijerinck confirmed Iwanowski's observations, and since 

 then numerous observations have been made. 



In M-Ai^ .—Y ellow fever virus by Reed and Carroll in 1901, 

 destroyed if heated to 53° C. for ten minutes; Molluscum con- 

 tagiosum, by JuJius Very in 1905; Dengue fever by Ashburn and 

 Craig in 1907; Three days' fever by Doerr in 1908; PoliomyeUtis 

 by Lentz, Landsteiner, Levaditi, Flexner, and Lewis in 1909; 

 Typhus by NicoUe in 1910; Measles by Goldberger and Anderson 

 in 1 91 1, as well as Trachoma, Scarlatina, Verruca vulgaris, and, 

 according to Nicolle, Influenza. 



Animals and Man.— Foo^^ and Mouth Disease by Loffler and 

 Frosch in 1898; Vaccinia by Siegel in 1905, Variola and Rabies. 



Besides these filterable viruses others have been found in diseases 

 of birds and one in plants. 



Cultivation. — ^The virus of pleuro-pneumonia of cattle, of fowl pest, 

 fowl diphtheria, epithelioma contagiosum, and of Novy's rat disease 

 have been cultivated. The organism of pleuro-pneumonia is a very 

 small spirochgete, but the organisms of the other disease are only 

 evident by the fact of their infectivity in subcultures carried beyond 

 any reasonable limit of dilution from the original. 



Immunity. — -In nearly every case the immunity produced by a 

 filterable virus is complete and of long duration. 



Secondary Infections.— Secondary infections with bacteria are 

 common, and it is thought that many of the so-called typical 

 symptoms of the disease may be due to the secondary agent. 



Methods of Infection.— The methods of infection are various: 

 (i) By blood-sucking insects — e.g., Stegomyia calopus and yellow 



