er, though in different ways.

Again, all the diseases under the second group, which seem to come from the
soil and from pastures, are in one sense contagious in that the virus may
be taken from a sick animal and inoculated directly, with positive results,
into a healthy animal. Other illustrations may be cited which show that
these old terms are not in themselves satisfactory. There are so many
conditions which enter into the process of infection that no single
classification will give a sufficiently correct or comprehensive idea of
it. These statements will be easily understood if the different infectious
diseases in the following pages are studied with reference to the way or
ways in which each disease may be contracted. Enough has been said,
therefore, to show that if we wish to make ourselves acquainted with the
dangers of any given disease, we must study it and not rely upon any single
work to tell the whole story.

Infectious diseases have, as a general rule, 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 with the malady. The
most common symptom of this class of diseases is fever. The severity of the
fever is measured by the temperature of the animal; this is readily and
accurately ascertainable by the clinical thermometer. (See Pl. III, fig.
1.) The other symptoms are variable and depend upon the particular organ or
organs most implicated. Loss of appetite, cessation of rumination and milk
secretion, and general dullness are symptoms quite invariably present in
most infectious diseases.

During the course of infectious diseases secondary diseases or
complications may arise which are largely caused by bacteria other than
those producing the original malady. These complications are often so
severe as to become fatal. In general it may be stated that they are due to
filthy surroundings, and hence cleanliness may become an important aid to
recovery.

The treatment of infectious diseases is given under each malady so far as
this is allowable or advisable. These diseases are not, as a rule, amenable
to treatment. When the symptoms have once appeared the 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 use