2 F. M. BURNET 



An animal virus is a replicating agent or microorganism which is capable of 

 growth and replication only within the living cells of some warm-blooded verte- 

 brate, and which is composed of, or contains, protein and nucleic acid carrying 

 patterns of specificity distinct from any produced under the genetic control of the 

 host alone. 



The first difficulty arises over the fact that this definition would include 

 rickettsiae which, by convention, are excluded from the group, although the 

 rather closely related agents of the psittacosis group are always known as 

 viruses. One good reason for this is the undoubted resemblance in some 

 aspects between the psittacosis viruses and the poxviruses. Both produce, for 

 instance, a lrpid-containing hemagglutinin. There are probably some viro- 

 logists who would regard all three groups, rickettsiae, psittacosis group, and 

 poxviruses, as small microorganisms of obligate intracellular parasitism but 

 in all other respects differing from "true" viruses. However, it is a historical 

 fact that most of the fundamental concepts of virology arose from the study 

 of the variola-cowpox-vaccinia system; if a systematist were seeking a type 

 species for the whole assemblage, he would be compelled to choose vaccinia 

 virus. By one means or another, our definition must include the poxviruses. 



In attempts to make a reasonably satisfactory compromise, a number of 

 possible additions to the definition have been suggested: 



1. The average value for the longest diameter of the infective particles 

 does not exceed 300 m/x (millimicrons). 



2. A virus is insusceptible to the action of antibiotics of the tetracycline, 

 penicillin, and sulfonamide groups. 



3. Viruses show no evidence of any intrinsic intermediary metabolism. 



4. An eclipse phase can always be recognized in the course of virus infection. 



5. Lwoff (see Chapter 5, Volume II) suggests that the term virus should 

 be confined to viruses having only one kind of nucleic acid. 



None of these is particularly satisfactory in practice. The quality suggested 

 may be one requiring elaborate experimentation to establish, while other 

 qualities may be unsuitable for application to certain strains. Some influenza 

 strains, for instance, may produce infectious filamentous structures up to 

 10/u, long, while sulfonamide- or penicillin-resistant strains of psittacosis 

 group viruses can be obtained. 



Despite Lwoff 's contention that a virus is a virus and not to be thought of 

 as a degenerated bacterium or anything else, it is very difficult to feel in any 

 way confident that in the animal viruses we are dealing with a group of bio- 

 logically related agents. In inviting contributors to write on this or that 

 aspect of the animal viruses, the editors felt it necessary to ask that, in 

 general, three viruses should be considered as prototypes — vaccinia, influenza 

 A, and polioviruses. There were two obvious reasons for this: the first, that it 

 was essential that at least three major types of virus should be considered 



