Viruses 393 



derived from the Latin and meaning a slimy or poisonous liquid. 

 Virus also means anything that poisons the mind or the soul. 



It was the work of Loeffler and Froesch in 1897-1898 that first 

 demonstrated a virus disease of animals, namely, foot and mouth 

 disease of cattle. Three to four years later (1901) Walter Reed 

 discovered yellow fever virus affecting man. It was not until 1915, 

 however, that virus diseases of bacteria were announced by Twort. 

 His work was confirmed in 1917 by d'Herelle who named this 

 filterable agent bacteriophage (bacteria-eater). In 1930 phage, 

 the accepted abbreviation for bacteriophage, was considered simi- 

 lar to viruses affecting animals and higher plants. 



CLASSIFICATION 



The sixth edition of Bergei/s Manual (1948) describes these 

 transmissible, parasitic agents as follows: 



"Viruses are etiological agents of disease, typically of small size 

 and capable of passing filters that retain bacteria, increasing only 

 in the presence of living cells, giving rise to new strains by muta- 

 tion ... A considerable number of viruses have not proved filter- 

 able; it is nevertheless customary to include these viruses with 

 those known to be filterable, because of similarities in other attri- 

 butes and in the diseases induced. . . Cause diseases of bacteria, 

 plants and animals." 



CHARACTERISTICS OF VIRUSES 



In addition to being too small to be seen normally under a 

 compound microscope, ultramicroscopic viruses are characterized 

 by their extreme dependence upon living cells for their existence 

 and multiplication. While their mode of multiplication is not 

 known, it is assumed that they divide in a manner similar to that 

 employed by bacteria— binary fission. Hence, viruses are placed 

 under the class Schizomycetes in the provisional classification 

 scheme; 



By use of electron microscopy different viruses have been found 

 to be spherical, rod-like, oval, and tad-pole shaped, and they range 

 in size from about 10 millimicrons to nearly 300 millimicrons. 



