6 AN INTRODUCTION TO THE STUDY OF VIRUSES 
girl, going to milk the cows, infected them with the foot-and-mouth 
disease virus. 
There are one or two very infectious virus diseases of fowls in 
which transmission of the virus can be effected by both direct and 
indirect contact. Fowl-pest, which originated in the Dutch East 
Indies, is thought to have been brought into England in imported 
carcasses. The virus can be passed on by direct contact between 
healthy and infected birds through the medium of the material which 
is discharged from the beak and nostrils and of the droppings. In- 
fection can also be carried in dirty cages without the infected birds 
themselves being present. In fowl-pox direct contagion is most 
frequent, but a small wound is apparently essential for infection, 
such as abrasions of the mouth, which are common as the result of 
eating grit. Burnet found that virus placed in the drinking water 
for one month failed to infect pigeons unless some sharp grit was 
fed at the same time. The manner of transmission of fowl-pox makes 
an interesting comparison with the spread of certain plant viruses 
which are considered later. 
In infectious myxomatosis of domestic rabbits, the tissues, blood, 
tumour fluids, and nasal secretions are all extremely infectious and it 
is sufficient to introduce a healthy rabbit into a cage which has con- 
tained a diseased rabbit for the former to become infected. In the 
case of psittacosis, or parrot fever, infection has been reported in 
persons handling imported feathers. 
The viruses attacking caterpillars and causing the so-called polyhedral 
and wilt diseases are amongst the most infectious. Transmission 
seems to take place mostly by the mouth. The leaves of the food 
plant become contaminated either with the polyhedral bodies or 
with the virus itself, and when the leaves are eaten the virus is ingested 
at the same time. These viruses are also easily transmitted to cater- 
pillars through a small wound, either artificially or accidentally 
made. Transmission by mechanical contamination, both direct and 
indirect, occurs also in some of the plant viruses: potato virus X 
spreads in the field by mechanical contact between infected and healthy 
potato plants. The virus can only enter the plant through a wound, 
however small, and this is provided for by the knocking of one plant 
against its neighbour by the wind and the consequent breaking of 
leaf hairs or slight abrasions of the leaf surface. Tobacco necrosis virus 
affords an interesting example of the spread of a plant virus by indirect 
contact; this virus is a peculiar one in many ways and occurs in the 
