CHAPTER I 
ETIOLOGY, INFECTION AND SPECIFIC 
INFECTIOUS DISEASES 
Etiology. The development of the sciences of bacteriology and 
of protozodlogy has shown that a large number of the infectious 
diseases are the direct result of the invasion of the animal body by 
certain species of microdrganisms. A specific etiology which teaches 
that for each of the various epizodtics we have a single, definite 
cause is recognized and accepted by all pathologists. Although 
there are a number of distinct diseases for which such a specific agent 
has not been found, the evidence in the very nature of the maladies 
is conclusive that for each of them such an etiological factor exists. 
In studying the pathology of infectious diseases the idea of a 
definite and adequate cause should be kept in mind. For many 
years the etiology of these maladies was thought to be unfavorable 
environment, poor hygiene or insanitary surroundings. While 
these conditions may favor the spread of disease producing organisms, 
they cannot cause the malady. The etiology of each of the specific 
infectious diseases of animals is a definite virus which alone can pro- 
duce the trouble. For many of the infections and epizodtics the 
properties of the specific organism are well understood. For certain 
of the others, such as rinderpest and Foot-and-Mouth Disease, the 
exact nature of the virus is not understood but its location in the 
body of the infected animal is known. With the morbid tissues 
the disease can be produced in susceptible animals and without this 
definite infection, no matter what the surrounding conditions are, 
the disease cannot be made to develop. These facts argue against 
extraneous conditions as exciting causes. 
The mystery which formerly surrounded the origin, the course 
and the disappearance of epizoétics has in a large degree been cleared 
away; and in its place we are confronted with the problems involved 
in the life history and the possiblities of invading microdrganisms. 
In fact, during recent years the biological sciences have been brought 
into actual use by the pathologists. Etiology has become per- 
manently linked to microbiology so that in seeking for the specific 
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