VIRUSES AS THE CAUSE OF DISEASE                      171

The exact and quantitative experiments with filters have been made possible
by the high concentration in which certain viruses occur, and by opportunities
for determining the presence of the virus in different dilutions by inoculation of
susceptible small animals. The dilution of some fluids containing virus from the
animal body can be carried to 1 in 10⁶ or even higher when dealing with foot-and-
mouth disease, vaccina and some other diseases without depriving them of infec-
tivity.

The quandary arising from the very active and apparently vital functions of
virus particles in spite of their small size raises the question whether the accepted
definitions of life are universally applicable or whether some intermediate state
between what is called living and dead matter may not exist, as has been suggested
by Boycott.

Of the functions usually postulated for a living organism, assimilation appears
to be the most characteristic and indispensable. It is reasonable to assume that
the metabolism of an organism would be much simplified if it existed in a circulat-
ing medium which provided a constantly changing supply of materials resembling
its own components, such as might be afforded for an obligatory parasite living
inside the cells of its host. Such an existence would have very different require-
ments from a truly independent life.

Virus diseases are transmitted from one animal to another by very varied
means. Some, like canine distemper and certain influenza-like diseases of man, by
droplets in the breath, others like yellow and dengue fevers by the bites of insects,
others like louping-ill of sheep by the bites of blood-sucking ticks or of mites ;
again, the bite of the mammalian host is the usual mode of infection with rabies,
but for many others the method of transmission is still uncertain. In these res-
pects they do not differ from diseases due to bacteria.

It is characteristic of many diseases that, although the initial infection is caused
by a virus, many of the symptoms and complications are due to secondary infec-
tions with bacteria, and this is notably the case in the influenza-like group in man
in swine fever, canine distemper and swine influenza.

A virus may become remarkably adapted and sometimes permanently
attenuated, when transferred to a new host, as is well instanced in the change
of the virus of small-pox to vaccinia in cattle and rabbits, and of the rabies virus
in the rabbit.

The period of resistance shown by the host following an attack of disease is
sometimes very prolonged even life-long after small-pox, varicella, yellow fever
and canine distemper, but in some other cases the protection afforded is of com-
paratively short duration, in foot-and-mouth disease usually for one to two years,
whereas frequently recurring attacks due to the virus of herpes labialis are common.