42 READINGS IN BIOLOGICAL SCIENCE 



We may define a virus as an infective agent below the size limit of 

 microscopic determination which passes through the finest made filters. 

 They are obligate parasites: no saprophytic forms are known. This is not 

 surprising, is it, since symptoms are the only means of recognizing them? 

 They can not grow and multiply in artificial culture media, but in tissue 

 culture, specific for the virus, the infective agent has been developed. For 

 immunization viruses are propagated by serial injection of animals. Their 

 behavior is very much like that of a living organism. 



An open mind is necessary in regard to the nature of viruses. One of 

 the smallest known viruses causes foot and mouth disease. It is only large 

 enough to hold a few dozen protein molecules. Is this consistent with life? 

 Would it be consistent if smaller viruses were discovered? Does not their 

 minute size preclude their being alive? It must be remembered that no 

 virus has a characteristic form or the ability to assimilate Hfeless matter. 

 Are not form and assimilation two of the chief attributes of life? These 

 questions can not be answered, for we do not know whether there is a 

 definite boundary line between the living and the non-living. 



As a virus is dependent on living cells for its development, does not this 

 suggest that they may be derivatives of those cells, an enzyme or catalyst, 

 for example? Catalysts effect a chemical change. The viruses then may have 

 the peculiar property of stimulating healthy normal cells to reproduce 

 more virus substance. The living characteristics that are possessed by a 

 virus are shown only when the virus is associated with living tissue, namely, 

 metabolic assimilation of heterogeneous substances, adaptation and re- 

 production. 



On the other hand, the principles of proof that a particular species of 

 living organism is the cause of a specific disease is stated in Koch's postu- 

 lates. First, the causal organism must be found in all cases of the disease; 

 secondarily, it must be grown in successive pure cultures outside the 

 body; thirdly, the cultures must be able to reproduce the disease in sus- 

 ceptible laboratory animals or plants, and lastly, the organism must be 

 recovered from the artificially infected host in pure culture. No doubt if 

 viruses were living they would follow these postulates. They do not 

 satisfy number two, namely, cultivation outside the tissue of the host. 



Most of the knowledge about viruses has been gained through the study 

 of tobacco mosaic virus. It is the oldest known. It was first described in 

 1857, but its filterability was not discovered until 1892. It was then dis- 

 covered that the extracted juice of a tobacco plant affected with mosaic 

 would infect a healthy plant if placed into its tissues or rubbed onto the 

 leaf hairs, even after it had been filtered through a Chamberland filter. 



Tobacco mosaic is the most infectious of all virus diseases. Even when 

 dried and ground into a powder, diseased leaves will still have the property 

 of infectability after months of desiccation. The virus may be extracted 

 by ether, chloroform, carbon tetrachloride, toluene or acetone without 



