that the serum from affected lungs retains
its virulence and may be used successfully for inoculation weeks or months
after the death of the animal from which it was taken. This is
particularly the case when this liquid is hermetically sealed in glass
tubes. Other investigators state that they have successfully infected
cattle by placing, in the nostrils, sponges or pledgets of cotton saturated
with such serum. Cattle have also, according to the best evidence
obtainable, been infected from the clothing of attendants, from horns used
in drenching, and from smelling about wagons which have been used to
transport affected carcasses. In the work of eradicating pleuropneumonia
from the United States many stables were found in which the disease would
appear and reappear after the slaughter of affected herds, and in spite of
any precautions which were adopted. These were always old stables, with
woodwork in a decaying condition and with floors underlaid with filth which
could not be thoroughly removed or disinfected. In every one of these cases
the destruction of the stable, the burning of the lumber of which it was
constructed, the removal of the accumulations beneath the floors, and
thorough disinfection, prevented the recurrence of the plague in new
stables built upon the same premises. This experience conclusively shows
that under certain conditions, at least, stables may retain the infection
for a considerable time, and that when restocked the disease may break out
again from such infection.

As a rule, however, the disease is acquired by a healthy animal being near
an affected one and receiving the contagion direct. Affected animals may
give off the contagion in the early stages of the disease before the
symptoms are apparent to the observer; also, they may retain this
infectious character, if they survive the attack, for six months and
probably for a year after all symptoms of the disease have disappeared.

_Incubation._--The time which elapses between exposure to the contagion of
pleuropneumonia and the first appearance of the symptoms of this disease
varies greatly with different individuals and with different outbreaks of
the disease. Ordinarily the symptoms of disease make their appearance
within three to six weeks after exposure; they may be observed, however,
within two weeks or they may not become apparent until nearly or quite
three months. It is this long period of incubation and the great length of
time that