ORDER II. VIRALES BREED, MURRAY AND 

 KITCHENS, 1944.* 



(Jour. Bact., J^T , 1944, 421.) 



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 mutation. A considerable number of plant viruses have not been proved filterable; 

 it is nevertheless customary to include these viruses with those known to be filterable be- 

 cause of similarities in other attributes and in the diseases induced. Some not known to be 

 filterable are inoculable only by special techniques, as by grafting or by the use of insect 

 vectors, and suitable methods for testing their filterability have not been developed; more- 

 over, it is not certain that so simple a criterion as size measured in terms of filterability will 

 prove to be an adequate indicator of the limits of the natural group. Viruses cause diseases 

 of bacteria, plants and animals. 



Our incomplete knowledge of the entities known as viruses has made their classification, 

 and consequently their nomenclature, a difficult matter. It is difficult to describe viruses 

 adequately because of their small size and because they are not cultivable. Electron micros- 

 copy has enabled a determination of the size and morphology of some of the viruses. Like- 

 wise, serological methods have been developed which are proving to be useful in distinguish- 

 ing between different species and types of viruses, but in many cases these methods have 

 not been applied. 



The usual characteristic that permits recognition of viruses is their capacity to produce 

 specific diseases. As indicated in the previous edition of the Manual (6th ed., 1948, 1127), 

 three constituent groups of viruses have come to be recognized, and to some extent named 

 and classified, through the largely separate efforts of bacteriologists, animal pathologists 

 and plant pathologists. Taxonomic overlapping of the three groups, viruses affecting bac- 

 teria, viruses having human and other animal hosts and viruses invading higher plants, can 

 hardly be justified as yet by available evidence. Nevertheless, it has been shown that a 

 single virus may multiply within, and cause morphological changes in, both a plant host 

 and an insect vector (Littau and Maramorosch, Virology, 2, 1956, 128). This seems to dispose 

 of the thought that adaptation to one environment necessarily precludes the utilization of 

 other sources for the materials needed for growth and multiplication. 



Because of the difficulties involved in preparing adequate descriptions of species of 

 viruses, many investigators have felt it undesirable to use binomials according to the Lin- 

 nean system and therefore have proposed numbering or lettering systems for species and 

 subspecies of viruses (see Johnson, Wis. Agr. Exp. Sta. Res. Bull. 76, 1927; Proc. Sixth 

 Internat. Bot. Cong. (Amsterdam), 2, 1935, 193; and Smith, Textb. of Plant Virus Dis., 

 Philadelphia, 1937, 615 pp). These sj^stems have made it difficult for persons other than 



* Prepared by the Editorial Board, March, 1956, from a tentative outline by Dr. Robert 

 S. Breed, January, 1956. Reviewed by Dr. C. H. Andrewes, Medical Research Council, 

 National Institute for Medical Research, London, England, Dr. F. O. Holmes, The Rocke- 

 feller Institute for Medical Research, New York, N. Y., and Dr. E. A. Steinhaus, Agri- 

 cultural Experiment Station, University of California, Berkeley, California, March, 1956. 



985 



