INFECTIOUS DISEASES. 
By RusH SHippEN HvuIvEeKoper, M. D., Vet. 
[Revised by A. Eichhorn, D. V. 8.] 
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 mor- 
bid 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 infec- 
tious diseases in which the causative agents have never been success- 
fully 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, small- 
pox, 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 
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