m, when from some reason its virulence increases.
It assumes this form also when, from reasons of rural economy and
commerce, large numbers of young and more susceptible animals are
exposed to its contagion.

_Etiology._--The experiments of Dieckerhoff many years ago proved that
the disease may be transmitted to healthy animals by intravenous
injection of warm blood from affected horses.

Further investigations revealed the fact that blood from affected
horses, even when passed through porcelain filters, may transmit the
disease, thereby proving that the causative agent belongs to the
so-called filterable viruses. This has been further substantiated by
Gaffky, who showed in his recent experiments that the disease may be
transmitted with defibrinated as well as with filtered blood, in which
cases the typical form of influenza developed in inoculated animals in
from five to six days. These findings were also substantiated by Basset.
Further observations have also proved that apparently recovered animals
may harbor the infection for a long time and still be capable of
transmitting the disease. Such virus carriers are no doubt responsible
for numerous outbreaks of this disease when, in a locality free from
the disease, it certainly appears after the introduction of an
apparently healthy animal.

As one attack is usually self-protective, numbers of old horses, having
had an earlier attack, are not capable of contracting it again; but,
aside from this, young horses, especially those about four or five years
of age, are much more predisposed to be attacked, while the older ones,
even if they have not had the disease, are less liable to it. Again, the
former age is that in which the horse is brought from the farm, where it
has been free from the risk of exposure, and is sold to pass through the
stables of the country taverns, the dirty, infected railway cars, and
the foul stockyards and damp stables of dealers in our large cities.
Overfed, fat, young horses which have just come through the sales
stables are much more susceptible to contagion than the same horses are
after a few months of steady work.

Pilger, in 1805, was the first to recognize infection as the direct
cause of the disease. Roll and others studied the contagiousness of
influenza, and, finding it so much more virulent and permanent in old
stables than elsewhere, classed it as a "stall miasm." The contagion
will remain in the straw bedding and droppings of the