8 DIFFERENTIATION 
infections following accidental injuries or surgical operations, al- 
though there are exceptions. Again, there is usually a difference 
in the mode of infection. The virus of an epizodtic disease is ordi- 
narily introduced through the digestive or respiratory tract or by 
means of insects, while in wound infection the virus is introduced, 
as the term implies, through the injured integument or mucosa. 
The differential characters of a specific infectious or epizodtic 
disease. It is very important not to mistake for an infectious 
disease some form of body disturbance due to another cause. Ani- 
mals often suffer from improper food and the conditions of life under 
which they are compelled to live. It frequently happens that when 
all of the animals in a given herd are subjected to like conditions of 
life, a number of them, perhaps all, will manifest simultaneously 
very similar symptoms and more or less of them die. Such an oc- 
currence often gives rise to the supposition that the cause of death 
is some specific infection. Deaths from such causes or under such 
conditions should be carefully distinguished from an epizoétic. 
In differentiating a non-infectious disorder from a specific disease, 
it is important, and usually sufficient, to take into account the 
appended characteristics of an infectious disease. 
Cause. An infectious disease is produced by a specific virus. 
This necessitates as the first requisite an exposure to and an infection 
with the specific organism. Ordinarily but a few animals in a herd 
are infected simultaneously. 
Period of incubation. The infection must be followed by a certain 
period of incubation before the development of symptoms. This 
is the time necessary for the invading microdérganisms to become 
established in the body and to bring about the first symptoms of 
the disease. According to Vaughn it is the time required for the 
infecting organism to sensitize the tissues. The incubation period 
varies in different diseases, and, to a certain degree, in the same 
disease, according to the mode of infection and the resistance of the 
individual. Usually, however, the incubation period of a given 
disease. is practically the same for all individuals of the same species 
when subjected to the same mode of infection. Exceptions may 
occur. 
Lesions. The morbid anatomy of an infectious disease is usually 
nearly the same in animals suffering in the same outbreak. Each 
pathogenic organism brings about tissue changes more or less peculiar 
