I. THE ORIGIN OF VIRUSES 



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found, for example, that infectious Newcastle disease virus particles 

 produce extensive pulmonary consolidation in the mouse in the absence 

 of demonstrable virus multiplication, the lesion being indistinguishable 

 from those of influenza A virus infection. It may be possible to 

 regard lysozyme or haemolysin as virus-like agent which, like New- 

 castle virus in this example, can cause a remarkable damage in the 

 cell without multiplication. 



The injurious action of viruses upon the host cells may probably 

 be effective only during the process of the virus multiplication ; with 

 the finish of the process the host cells will be released from the suffer- 

 ing and will be able to behave like apparently healthy cells, unless the 

 structure itself which has been provided by the virus is injurious to 

 them. A virus as a rule can be detected in abundance in plant cells 

 which have been recovered from the infection by the virus. Also in 

 the case of animals, certain viruses are reported to be detected in some 

 tissues long after the recovery from the virus infections. 



The injurious effect, as above stated, is not necessarily associated 

 with the virus multiplication, a fact which is clearly shown in the 

 term of "inapparent" or "subclinical" infection, which is often regar- 

 ded as a characteristic manifestation of viruses. Thus it is generally 

 accepced that viruses tend to cause infections that do not give rise to 

 clinically evident diseases. 



It will, therefore, naturally follow that fragments or particles of 

 some protoplasm capable of producing replica in other cells may fail 

 to be realized of their existence because of their inability to cause 

 any injurious effect, and that, even when their existence was acknow- 

 ledged, they would not be called viruses. The well known principle 

 found by Avery et al (16) with pneumococci can be regarded as one of 

 such "viruses." This "virus" is produced from virulent, capsulated 

 pneumococci and can convert avirulent, non-capsulated culture to the 

 virulent type. The possible mode of action of this agent is illustrated 

 in Fig. 17. 



Transforming agent 



Fig. 17. The change of R-type pneumococcus into S-type by the trans- 

 forming agent produced by S-type. 



