EPIDEMIC INFLUENZA 105 



The next step was to define the living agent more 

 exactly and to attempt its cultivation in the laboratory. 

 It was soon found to be so minute that it readily passed 

 through earthenware filters impervious to ordinary 

 bacteria. In this way it could be separated from other 

 microbes in the human nasal washings or in affected 

 rabbits' lungs. Filtered nasal washings from influenza 

 patients and filtered lung suspensions produced the 

 typical train of effects in rabbits and thus proved that 

 ordinary bacteria were not involved in the process. The 

 specific microbe, which as yet had not been seen but 

 whose presence was clearly indicated by the animal 

 experiments, was a "filter-passer." Now although very 

 few filter-passing microbes have been identified, the 

 group, in general, has certain well-known characteristics 

 which this virus was found to share. For example, 

 although it was readily killed by heat at 133-140 F., 

 temperatures often used in pasteurization, it was resis- 

 tant to drying or freezing and could withstand the 

 action of 50 per cent, glycerine for periods up to nine 

 months. When animal tissues containing it were con- 

 taminated by moulds or bacteria, the virus still survived. 



Another noteworthy effect of this active agent early 

 claimed attention. When unfiltered nasal washings 

 from influenza patients were injected into rabbits* lungs, 

 other microbic residents of the nose and throat were 

 likewise deposited. Ordinarily such bacteria do not 

 do any damage under these conditions but are over- 

 powered by the active protective mechanisms of the 

 body. But in the presence of the primary injury caused 

 by the influenzal agent these bacteria were sometimes 



