ministered
subcutaneously by means of a syringe, but the quantity of the vaccine to
be injected and the number of doses to be used should be left to the
judgment of a competent veterinarian.

INFECTIOUS DISEASES.

By RUSH SHIPPEN HUIDEKOPER, M. D., Vet.

[Revised by A. Eichhorn, D. V. S.]

GENERAL DISCUSSION.

An infectious disease may be defined as any malady caused by the
introduction into the body of minute organisms of the vegetable or
animal kingdom which have the power to multiply indefinitely and set
free certain peculiar poisons which are chiefly responsible for morbid
changes. Nearly all diseases of animals for which a definite cause may
be attributed are caused by bacteria; such are tuberculosis, anthrax,
blackleg, lockjaw, and others. There are some diseases, as, for
instance, Texas fever and rabies, which are caused by a minute animal
parasite known as protozoa, while others again, like lumpy jaw and
aspergillosis, are caused by fungi. Besides there are infectious
diseases in which the causative agents have never been successfully
isolated, as they are so small that they can not be detected by the aid
of the most powerful microscope, and accordingly they are termed as
ultravisible viruses. Hog cholera, foot-and-mouth disease, smallpox, and
others belong to this group.

Bacteria may be defined as very minute unicellular organisms of
plantlike character. They multiply either by simple division or by spore
formation, the latter usually taking place when the conditions
pertaining to the growth of the bacteria become unfavorable. The spores
are much more resistant to destruction than the bacteria which produce
them.

Another group of parasites producing disease is known as protozoa. These
are more complex than bacteria, and their artificial cultivation is also
much more difficult than is the case with the bacterial parasites. Of
the representatives of this group, causing disease in animals, are the
trypanosomes, which are the causative factors of dourine and surra, and
the piroplasma, which induce Texas fever in cattle and malaria or
biliary fever of horses. There are also disease-producing fungi which
are responsible for certain affections in horses; among these the most
important are mycotic lymphangitis, or sporotrichosis, and
streptotrichosis.

The introduction of the infection may take place in various ways. The
most frequent method is by ingestion. Further, the entrance of the
germs may