VIRUSES 



FIGURE 1 - TOBACCO LEAF SHOWING SYMPTOUS 

 OF TOBACCO MOSAIC. BRIGHT YELLOW SPOT IS RE- 

 SULT OF MUTATION. (L.O.Kunkel, Ann. Rep. Quebec 

 Soc. Proc. Plants 22, 2^ (1934J ;. 



One of the most outstanding properties of the causative virus is its extreme 

 stability. Tobacco mosaic virus is so stable that it is able to survive the 

 processing given commercial tobacco. It can be obtained fairly regularly by 

 merely rubbing the leaf of a healthy tobacco plant with a moistened cigarette. 



Beijerinck's revelation, that disease could be caused by agents differ- 

 ent from ordinary bacteria, was almost as shocking as Pasteur's earlier dis- 

 covery. It inspired a search for other viruses as the possible causative agent 

 of other diseases of plants and animals . The result was that many virus dis- 

 eases were discovered, and many of the properties of the filterable agents 

 responsible were determined. Included among these virus diseases are small pox, 

 sleeping sickness, yellow fever, InfEintlle paralysis, measles, mumps, dengue, 

 influenza, and fever blisters in man, foot and mouth disease and pox of cattle, 

 rabies and distemper of dogs, horse encephalitis, hog cholera, pox and various 

 tumors of fowl, many mosaic and yellow diseases of plants, and even the con- 

 dition responsible for the unusual beauty of some tulips - a disease called 

 tulip break. The agents responsible are usually fairly specific, for they 

 cause disease only in a few hosts and are harmless in all others . Within the 

 cells of the specific host, these viruses sure multiplied, and occasionally 



