168 THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [VI, II
shown to cause a disease of animals, was discovered in 1898 by Loeffler and Frosch.
It passes through finer filters than any other known virus, so that no question of
visible particles has arisen, since massed granules or inclusion bodies have not been
observed. The most essential qualifying characteristic of a virus has ever since
been its filterability.
Most viruses remain active after being dried over sulphuric acid, and some
are more resistant to alcohol and certain other disinfectants than bacteria. Many
viruses are present in the tissue juices in high concentration, and such suspensions
in liquids can still prove infective when diluted 1 in 10 5 or 1 in 10 6 . The resem-
blance in many respects of a virus to an excessively minute bacterium has led to
the belief in their similar nature which is now held by most pathologists. It
must be admitted, however, that part of the argument is based on analogy, since
viruses cannot be subjected to the same tests as bacteria to prove that they are
living agents causing disease.
Besides the small size of the ultimate particles of a virus and the resulting
absence of a recognisable differentiating morphology, there are certain other pe-
culiarities distinguishing these two classes of agents. Bacteria, with few exceptions
can be propagated on sterilised artificial culture media, and can be obtained in
pure culture by the method introduced by Koch of selecting single colonies grown
on a solid sterile medium. By this means their infective and other activities can
be examined without the risk of contamination with substances derived from the
host. A virus, on the other hand, in most cases requires the presence of living
cells of the host to enable it to multiply, and it often appears to grow only or chiefly
inside the cells of the animal tissues. Artificial culture of a virus can, however,
very often be maintained in pieces of animal tissue kept alive and growing apart
from the body. The virus of fowl-pox, vaccinia or vesicular stomatitis of the
horse and of some other diseases can also be propagated in the living embryo in
an incubated hen s egg.
The fact that tissues of a host are needed to enable viruses to multiply has led
to the suggestion that the virus may not necessarily be alive but may only serve
as a stimulus to the host cells, causing them to reproduce the virus, and that the
particles seen in a suspension containing a virus and indeed the virus itself are
really products of the host.
This suspicion has been especially strong in the case of the infective transmis¬
sible sarcoma of fowls described by Rous, which can be reproduced by injecting a
filtered cell-free extract of the diseased tissues into a normal fowl. The resemblance
of avian sarcoma to other virus diseases extends to the recognition by Ledingham
and Gye, by the use of the high-speed centrifuge, of minute particles resembling
those of other viruses. The nature of these particles was, moreover, confirmed
by their reaction (agglutination with the blood serum of animals which had been
injected with the sarcoma), in the same way that similar reactions have been de-
monstrated with the elementary bodies from other virus diseases.