RECENT ADVANCES IN VIRUS RESEARCH ! 
By WENDELL M. STANLEY 
Professor of Biochemistry and Director of the Virus Laboratory 
University of California 
Viruses are small infectious agents that can cause disease in man, 
other animals, plants and bacteria. They range in size from about 
10 my, a size slightly smaller than that of certain protein molecules, in 
an almost continuous spectrum of sizes up to about 300 muy, a size 
slightly larger than that of certain accepted living organisms. A given 
virus can multiply and cause disease only when within the cells of 
certain specific living organisms. No virus has been found to repro- 
duce in the absence of living cells. During multiplication viruses 
occasionally change or mutate to form a new strain which in turn 
causes a new disease. Viruses were not discovered until 1892 when 
Iwanowski demonstrated that the causative agent of the mosaic disease 
of tobacco would pass through a filter that retained all known living 
organisms. Six years later Beijerinck proved that this agent was 
not an ordinary living organism and recognized it as a new type of 
infectious disease-producing agent—namely, a virus. The same 
year Loeffler and Frosch demonstrated that foot-and-mouth disease 
of cattle was caused by a virus. The discovery of the first virus 
disease of man, that of yellow fever, was made in 1901 by Reed and 
coworkers. 
Since the original discovery of the infectious, disease-producing 
agent known as tobacco mosaic virus, well over 300 different viruses 
capable of causing disease in man, animals, and plants have been 
discovered. Among the virus-induced diseases of man are smallpox, 
yellow fever, dengue fever, poliomyelitis, certain types of encephalitis, 
measles, mumps, influenza, virus pneumonia, and the common cold. 
Virus diseases of animals include hog cholera, cattle plague, foot-and- 
mouth disease of cattle, swamp fever of horses, equine encephalitis, 
rabies, fowl pox, Newcastle disease of chickens, fowl paralysis, and 
certain benign as well as malignant tumors of rabbits and mice. Plant 
virus diseases include tobacco mosaic, peach yellows, aster yellows, 
1 Talk presented at the Medal Day Meeting at The Franklin Institute, October 20, 1948. Reprinted 
by permission from Journal of the Franklin Institute, vol. 246, No. 6, December 1948. 
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