164 Perspecfives in Microbiology 



cern any clear pattern of effects. This is an area of inquiry 

 in which borderline results can be both encouraging and 

 deceptive. Minor extensions in survival time, even though 

 not attributable to chance on statistical grounds, may be 

 so dependent upon the experimental conditions employed 

 as to rob them of any real importance. 



Some mention has been made of the short interval that 

 seems to be available for the effective use of inhibitory 

 compounds after evidence of disease has appeared. The 

 idea that this interval is brief has arisen from studies with 

 viruses like influenza, which reproduce at a relatively rapid 

 rate (28). With pneumonia virus of mice, which multiplies 

 less rapidly, an effective interval of at least a day remains 

 after gross signs of disease have appeared (23). Possibly with 

 viruses that reproduce much more slowly, an even longer 

 interval for therapy may be found. 



At present, it seems probable that substances which in- 

 hibit virus reproduction may come to be more useful dur- 

 ing the incubation period than after disease has become 

 manifest. Investigations with a variety of animal viruses, 

 including pneumonia virus of mice (23, 30), mumps (20), 

 and influenza (35, 38), indicate that multiplication is more 

 effectively inhibited the earlier an inhibitory compound is 

 given. As would be expected from a compound that inhibits 

 the reproductive process, the lower the virus concentration 

 when the substance is given, the smaller is the final yield 

 of virus. If the final concentration can be kept below the 

 threshold value, recognizable disease does not appear. Al- 

 though use of inhibitory compounds during the incubation 

 period is not to be considered as therapeutic, it is also not 

 strictly prophylactic. The virus infection is not wholly pre- 

 vented but is diminished in extent through reduction in 

 the amount of virus reproduction. In effect, the infectious 

 process is held at the subclinical or inapparent level. The 

 advantage can be double, for not only may evident disease 



