CHAPTER I 
INTRODUCTORY 
Tuere is little doubt that virus diseases have existed since very early 
times, though it is not known for certain how early because of the 
lack of precise descriptions. Evidence has been advanced to show 
that smallpox existed in China as early as 1700 B.c., and Zinsser (1937) 
says that it is definite that smallpox was a common condition all 
through North Africa by the sixth century a.p. By the year 1000 
it was present in practically all European nations and was again and 
again reintroduced from the East by returning crusaders. By the 
middle of the sixteenth century it is clear that the entire world had 
become infected with the virus. Of this virus disease Zinsser speaks 
as follows— 
The smallpox epidemics of the subsequent two centuries, recurring 
whenever susceptible fuel had accumulated, were of an extent and severity 
of which it is hard for us to form any conception at the present time; and 
it is safe to say that this condition would still prevail, attacking each new 
generation, were it not for the single and simple procedure of Jennerian 
vaccination. 
This statement might well be taken to heart by those responsible 
for the repeal of the vaccination laws. 
The earliest reference to a condition in plants now known to be 
due to a virus infection is a description published in 1576 by Charles 
l’Ecluse of a variegation in the colour of tulips, which is now called 
“breaking” and is due to an aphis-transmitted virus. 
Tulips showing this type of “break” are figured in Theatrum Florae, 
published in 1662, and are thought to be the work of the painter 
Daniel Rabel. About 1670, in Traité des Tulipes, the first suggestion 
was made that the variegation in the flower colour might be due to 
a disease. 
Towards the end of the eighteenth century the “curl’’ disease of 
potatoes was much in evidence and the favourite explanation for it 
was the “degeneration” theory, which supposed the “curl” to be a 
kind of senile decay caused by long-continued vegetative propagation. 
We know now that this degeneration is due solely to the infiltration 
of viruses into the potato stocks and that, if properly protected from 
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