16 AN INTRODUCTION TO THE STUDY OF VIRUSES 
explain the spontaneous development of the disease in isolated farms 
and districts where no other contact with a source of infection can be 
discovered. The case would, however, be made still stronger if, 
firstly, it could be demonstrated under experimental conditions that 
a starling can actually infect a cow by virus carried on its feet or 
elsewhere, and, secondly, if a starling had ever been found with its 
body naturally contaminated by virus. 
The question of living reservoirs of infection is an important 
one in the virus field and it is of course intimately bound up with the 
question of symptomless carriers which is discussed elsewhere. The 
problem of virus reservoirs has been studied in the cases of jungle 
yellow fever and of equine encephalomyelitis. In an attempt to find 
out what jungle animals could act as reservoirs of the yellow fever 
virus more than two thousand wild animals were captured and 
inoculated. It was found that a surprising number of these animals 
were susceptible to the virus and could retain it in the blood for some 
time without showing any signs of illness. It is tentatively concluded 
from this that yellow fever is primarily a disease of jungle animals and 
the transmission of the virus from man to man by the mosquito 
Aédes aegypti is in the nature of a secondary cycle. 
The reservoirs of infection of equine encephalomyelitis virus do 
not seem to have been identified, though certain birds such as egrets 
have been suspected of acting as carriers of this virus. 
Viruses Which Spread by Unknown Means or Which do not 
Spread at All 
Among the plant viruses are several which spread by unknown 
methods; in some cases this lack of knowledge may be due merely to 
insufficient investigation of the problem and no doubt the mode of 
spread of all these viruses will be elucidated in time. However, in 
other cases, in spite of careful investigation the problem has not been 
solved. Attempts to find the insect vectors of tomato bushy stunt 
virus, of tomato black ring virus, and of wheat rosette virus, to quote 
some examples, have failed and yet there seems no doubt that these 
three viruses do spread in nature. The same applies to the viruses of 
lovage mosaic, Arabis mosaic and broken ringspot, but since these 
viruses have each been isolated from a single plant we have no evidence 
that they can be spread from plant to plant other than by artificial 
means. 
