200 A. LWOFF 



bacterium, not only the potential power of producing bacteriophage particle, 

 but also a specific bacterial immunity against homologous bacteriophages. 

 Finally, the prophage may sometimes also be responsible for an alteration 

 of the antigenic constitution of the bacterium. The prophage is a modifying 

 agent. 



This ability to modify their host is one aspect of viruses. In the case of the 

 lysogenic bacterium, the modification is not a disease in itself. It might be a 

 disease in the case of malignancy. 



In a virus-induced cancer, a normal cell is altered by an oncogenic virus. 

 The malignant cell continues to grow and to divide and, considered by itself 

 is healthy. A cell is, however, not an independent unit, but is a dependent 

 part of an organism. 



An organism controls the growth and multiplication of the normal cell but 

 not of the malignant one, which behaves as an independent unit. Its multi- 

 plication causes the death of the organism. The oncogenic virus, although it 

 only modifies a ceU, kills the organism and is therefore pathogenic. 



The action of a virus has to be considered with respect to the multicellular 

 organism as well as to the individual cell. What is a mere harmless alteration 

 at one level might be a fatal disease at another. A virus acting at the level of 

 the dependent and interdependent parts necessarily modifies the indepen- 

 dent whole. 



IX. Conclusions 



The student of viruses should remember a few essential principles. 



1. Viruses differ from organisms by some essential features: presence of 

 only one nucleic acid, multiplication as genetic material, reproduction from 

 genetic material, inability to grow and to undergo binary fission, absence of a 

 Lipmann system. 



2. Viruses differ from cellular organelles by the existence of an organized 

 infective phase. 



3. Infectivity of a virus is the ability to introduce its genetic material into a 

 cell. A viral infection is the introduction into a cell or an organism of an 

 entity able to multiply and to reproduce organized infective particles. 



4. Viruses may exist in three states: proviral, vegetative, and infective. 

 Reproduction, infectivity, and pathogenicity are never present together 

 during any one of the three phases. 



5. One of the characteristics of viruses is their power to impose a permanent 

 alteration of their host cell. 



6. The action of a virus has to be considered at the level of the dependent 

 parts of an organism, as well as at the level of the independent organism as 

 a whole. 



