10 DISSEMINATION 
jacoboea) may be mentioned. The non-infectious disorders are 
differentiated from the infectious ones largely by eliminating the 
characters of the latter and finding, if possible, the causative agent. 
The necessity for an early and positive diagnosis in all outbreaks 
of epizodtic disease, is to assure the enforcement of all possible 
measures to prevent its further spread. The essential problem for 
the practitioner in the presence of these diseases, is to restrict the 
number of cases to the individuals already infected. In order to 
do this, it is of much importance that modified or chronic cases 
should not escape detection as there is danger of their spreading 
the virus to susceptible animals. 
Dissemination of infectious diseases. Although the discussion 
of the means by which each of the various diseases are disseminated 
will be found under the descriptions of the individual affections, 
it is important to consider the general ways and means by which 
these different vital, causative factors are spread from an infected 
individual to a non-infected one in the same herd and also from one 
herd to another. Each virus is dependent for its perpetuation 
upon its escape from its host (sick or dead) to a susceptible one. 
As these organisms are without power of their own for such migra- 
tion, they are dependent upon other forces and carriers to take them. 
In explaining their spread, we must know how they escape from the 
infected individual, how they are carried from one individual to 
another and how they gain entrance to the bodies of healthy individ- 
uals. 
Escape of virus from infected animals. The infected organisms 
escape from the living body of the infected animal either with (a) the 
excreta, (b) the external discharge of ulcers or abscesses or both, 
and (c) the blood drawn by sucking insects. After the death of 
the host they can escape only after the disintegration of the dead 
body or by its being consumed as food by other animals or birds. 
The bacteria of several diseases may pass through the digestive tract 
of such animals uninjured. 
Dissemination of infecting organisms after they leave the body. 
Pathogenic bacteria are spread, after they escape from the body, in 
a number of ways. The following are the more common: 
They are carried on the hands, shoes or clothing of attendants, 
and on farm implements, such as shovels and hoes, in crates and cars 
that have contained the infected animals. 
