THE PROTOZOA 



isms. When it was discovered that organisms living as parasites are the 

 causative agents in many such diseases, the basis for the observed facts became 

 apparent. Still problematical, hov^ever, were the increasing number of in- 

 fectious diseases in which no parasitic organisms could be discovered. In 

 these cases it appeared that "something" was present which increased like an 

 organism during growth; this entity came to be called a virus. A virus was 

 shown to be something invisible, capable of passing through filters which 

 retained bacteria. It was further discovered that although a virus could be 

 transferred from cell to cell and continue its multiplication, it was unable 

 to grow or multiply except in the living cells in which it was found. 

 Diseases now known to be caused by viruses include the type of sleeping 

 sickness called encephalitis, as well as hog cholera, poliomyelitis, parrot 

 fever, smallpox, yellow fever, and a long list of plant diseases, notably the 



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ir 





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Fig. 8.26. The virus cau.sing influenza in man. The particles are 

 normally individual spherules, but in being prepared for this electron 

 micrograph many have clumped together or have collapsed on the 

 plastic film used to support the preparation. The line marked l>i 

 represents 1 micron (O.OUl mm) enlarged to the same degree as the 

 virus particles. (Electron micrograph courtesy D. E. Philpott.) 



261 



