io6 SCIENCE REMAKING THE WORLD 



able to multiply and cause severe pneumonias. The 

 similarity of these accidental infections in the experi- 

 mental animals to the secondary pneumonias in man 

 led to a series of experiments to put this significant train 

 of events to further test. 



These experiments proved that a decrease in the re- 

 sistance of the lungs to common bacteria was a charac- 

 teristic result of infection with the filterable virus. 

 After the lungs had been damaged by the influenzal 

 agent the other organisms were injected into the trachea 

 or into the blood stream, and from both sources the 

 common bacteria invaded the injured lungs and there 

 induced a typical pneumonia. To the normal lungs of 

 healthy or "control" animals the same doses of these 

 microbes were harmless. 



The first object of the investigation had now been 

 accomplished. An infectious agent, present only in the 

 nasal washings of influenza patients in the early hours 

 of the disease, had been transmitted to laboratory 

 animals and could now be studied under experimental 

 conditions. The presence of this virus induced changes 

 in the blood that are typical of the blood changes in 

 human influenza. And the lungs of the infected rabbits 

 were found to be the site of typical injuries, which pre- 

 disposed them to severe and fatal pneumonias. 



ARTIFICIAL CULTIVATION. From the beginning of 

 the investigation, while the first animal transmission 

 experiments were in progress, attempts were made to 

 isolate the active agent in artificial cultures outside the 

 body. For this purpose the usual methods of cultiva- 

 tion were discarded in favour of the particular methods 



