Chemical Inhibition of Viruses 163 



phase and the virus concentration in the lung is consider- 

 able, but also when some gross pneumonia has already ap- 

 peared. On administration of the compound, further multi- 

 plication of the agent is inhibited, and the amount of 

 pneumonia does not increase beyond that expected from 

 the preformed concentration of virus. In this particular 

 system, at least, some support apparently can be obtained 

 for the conjecture that a substance which inhibits virus re- 

 production may exert a therapeutic effect. 



Among the difficulties that have appeared, however, are 

 the following: With substances so far employed, the in- 

 terval between appearance of the disease and administra- 

 tion of the compound cannot be long if a beneficial effect 

 is to be achieved (23). Moreover, the identity of the virus 

 responsible for the disease needs to be known, for inhibi- 

 tory compounds may show striking selectivity for different 

 viruses in the same host tissue. As an example, K. pneu- 

 moniae polysaccharides have no inhibitory effect on the 

 reproduction of influenza viruses and do not modify the 

 course of pneumonia induced by these viruses in the mouse 

 (27, 30). This is in sharp contrast to the effects they pro- 

 duce in the same tissue on infections with pneumonia virus 

 of mice (27, 30). It may be that these different viruses do 

 not reproduce in precisely the same cells in the lung and, 

 therefore, that the host cell systems are not identical even 

 though present in a single tissue. But this does not affect 

 the practical aspects of the situation from the viewpoint of 

 the mouse. Under the conditions mentioned, administra- 

 tion of the polysaccharide can modify one virus-induced 

 pneumonia but not the others. 



Whether other substances with inhibitory activity will 

 show a similar degree of selectivity with different animal 

 viruses remains to be seen. The number of potentially use- 

 ful substances so far discovered is discouragingly small. 

 Only a very few such substances have been closely studied 

 with a variety of different viruses, and it is too early to dis- 



