PROBLEM 3. Rece??t Discoveries Change Our Ideas About Disease 339 



Equally startling experiments have 

 been performed at the Rockefeller Insti- 

 tute for Medical Research in New York. 

 A vaccine against pneumonia was made 

 chemically in a test tube. The substances 

 used were white of egg (a protein called 

 albumen) and sawdust, which contains 

 the carbohydrate cellulose. This new 

 vaccine has been used on animals and 

 proved to be as effective on them as the 

 vaccine made of dead pneumonia germs. 

 To sum up ^\■hat you have read do 

 Exercise i. 



A new field — viruses. Though the ex- 

 istence of viruses has been known for 

 about 50 years, we know comparatively 

 little about them. But this much is cer- 

 tain. They are so extremely minute that 

 they can pass through porcelain or stone 

 filters. That is why they are often spoken 



of as "filterable viruses." It has been esti- 

 mated that the smaller ones among them 

 would have to be magnified a thousand 

 times or more to be the size of a typhoid 

 germ. The electron microscope has en- 

 abled us to see images of viruses. 



Some viruses seem to be very resistant 

 to cold. Influenza virus has been kept for 

 several months packed in dry ice at a 

 temperature of -108° F, and when after 

 this treatment it was diluted one to a mil- 

 lion it was still virulent enough to kill 

 experimental animals. 



Viruses are so intimately associated 

 with the living cell that they cannot exist 

 away from it. That is, we cannot raise 

 viruses on nutrient agar. They will grow 

 only on living tissue cells. Keeping tissue 

 cells alive outside the body is a fairly 

 recent discovery. It was early in this 



i 



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Fig. 302 Influenza virus photographed with an electron microscope. (See page n for 

 a picture of an electron ynicroscope.) TJje white spheres which look like balls of cot- 

 ton are the virus particles. They are magnified about 6o,ouo times. In what ways are 

 these virus particles different from bacteria? Why are viruses difficult to study? What 

 are some of the other diseases caused by viruses? (r. c. williams and r. w. wycoff) 



