2 AN INTRODUCTION TO THE STUDY OF VIRUSES 
virus infection, potatoes can be propagated indefinitely by vegetative 
methods. 
It was about this time also, 1796 to be exact, that Jenner made his 
first experiment in vaccination against smallpox by inoculating the 
arm of a boy named James Phipps from a pustule on the hand of a 
young woman who had contracted it while milking the cows. Later, 
the boy was inoculated with the smallpox virus without effect. 
In 1886 Mayer described a disease of the tobacco plant which he 
called Mosaikkrankheit and showed that this “mosaic” disease could 
be communicated to a healthy tobacco plant by inoculation with the 
sap of the infected plant. The first’ scientific demonstration of the 
existence of a virus was made in 1892 by the Russian botanist, 
Iwanowsky, who found that the sap from a mosaic-diseased tobacco 
plant was still infectious after passage of a bacteria-proof filter candle. 
About six years later, two German workers, Loeffler and Frosch, 
found that foot-and-mouth disease of cattle was also caused by a 
filter-passing virus. They made the observation, interesting for that 
time, that the disease could not be due to a poison because of the 
infinitesimal quantity which was capable of causing infection. 
The next discoveries concerned the mode of spread of certain 
viruses and showed the relationship existing between viruses and 
insects. Probably the first to prove experimentally that some relation- 
ship existed between viruses and insects was a Japanese farmer, 
Hashimoto, who worked in 1894 with the dwarf disease of rice and 
a leaf-hopper. These studies were continued by various Japanese 
workers and in 1906 the Imperial Agricultural Experiment Station of. 
Japan proved that the leaf-hopper was not itself the cause of the 
disease but carried to the plant something which caused it. 
In America, during the years 1909-15, various workers pointed 
to some connexion between the curly-top disease of sugar-beet and 
the leaf-hopper, Eutettix tenella. Finally, in 1915, Smith and Boncquet 
showed that a single insect, after feeding on a diseased sugar-beet, 
could infect a healthy sugar-beet if allowed to feed on it for five 
minutes. It was in 1900 that Walter Reed demonstrated that the virus 
of yellow fever was transmitted by a mosquito. 
A chance observation made in 1896 by a bacteriologist, Hankin by 
name, was to be repeated nearly twenty years later as a discovery of 
great scientific importance. While making a bacteriological examin- 
ation of the water of the river Jumna, Hankin found that, whereas 
immediately below Agra the water contained more than 100,000 
