BACTERIOPHAGE AS A MODEL OF HOST- VIRUS RELATIONSHIP 197 



Some virologists may find it difficult to decide whether a middle-sized 

 agent is a bacterium or a virus, but if they are willing to look carefully at the 

 thing in question for the characteristic features of viruses the decision should 

 always be possible. As already mentioned we do not know what the 

 ancestors of viruses have been and it remains possible that some have evolved 

 from cellular constituents, other from parasitic microorganisms. A theoretical 

 possibility has therefore to be considered: it may turn out that an infectious 

 entity will sometime be discovered possessmg the features of a virus during 

 one phase of its life cycle, for example, multiphcation m the form of its 

 genetic material, and the features of a microorganism during another phase, 

 for example, ability to grow. Such a creature would be highly interesting, 

 but it remains a hypothesis. 



For the time being, making use of the criteria which have been 

 proposed, it is possible to decide whether an uifectious agent or whether a 

 given particle, is or is not a virus. A virus is neither an organism nor an 

 organelle. A virus is a virus. 



VII. Are Viral Diseases Always Infectious? 



The virus has been defined by a few biochemical and physiological featm-es 

 associated with two properties, or potential properties, namely, infectivity 

 and pathogenicity. The very title of this section might therefore seem strange. 



Classically, an infectious disease is a disease which is started by an 

 infectious entity and ends with more infectious entities. A bacteriophage 

 infects a bacterium, the bacterium lyses and liberates 100 phage particles: 

 this is a ty]3ical infectious disease (see Table III). 



TABLE III 



Some Diseases of the Bacterial Metabolism ° 



" "Infectious disease" is restricted to those cases where the disease is started by an 

 infectious particle and ends in infectious particles. 



