occur by inhalation, skin abrasions, wounds of any kind,
through the genital organs, and at times also through the milk ducts of
the teats. As a general rule infectious diseases have a period of
incubation which comprises the time elapsing between the exposure to the
infection and the actual appearance of the disease. This period varies
in the different diseases.

The treatment of infectious diseases is, as a rule, unsatisfactory. When
the symptoms have once appeared a disease is liable to run its course in
spite of treatment, and if it is one from which animals usually recover,
all that can be done is to put them into the most favorable
surroundings. Many infectious diseases lead sooner or later to death;
treatment is useless so far as the sick animals are concerned, and it
may be worse than useless for those not yet affected. All animals
suffering with infectious diseases are more or less directly a menace to
all others. They represent for the time being manufactories of disease
germs, and they are giving them off more or less abundantly during the
period of disease. They may infect others directly or they may scatter
the virus about and the surroundings may become the future source of
infection.

Therefore, in the control of infectious diseases prevention is the most
important procedure. The isolation or segregation of healthy animals
from infected ones should be primarily considered, and if at any time an
animal manifests the symptoms of an infectious disease it is essential
to protect the others from such a source of danger. In some of the
infectious diseases it may become advisable to kill the infected animals
in order to avoid the spread of the disease. This is especially
important in diseases which are slow in their course, such as
tuberculosis. At times when diseases appear in a country where they have
not been prevalent it becomes advisable and necessary to protect the
healthy herds by the slaughter of all the infected animals. Pursuance of
this policy has resulted in control of the foot-and-mouth disease, and
has proved to be a very satisfactory method of eradication.

DISINFECTION.

Disinfection is a very important phase in the control of infectious
disease. This consists in the use of certain substances which possess
the power to destroy bacteria or their spores, or both. The cheapest and
most available for animal diseases are ordinary freshly slaked lime, or
unslaked lime in powder form, chlorid of l