DISSEMINATION OF VIRUS 11 
They are carried in streams receiving the excreta or disintegrating 
dead bodies of the infected animals. 
They are scattered with the excreta of birds that feed upon dead 
carcasses. Other animals, such as dogs and foxes, are also charged 
with the scattering of the virus by the same method. 
The virus may be carried from one herd or flock to another (a) 
by animals suffering with chronic forms of the disease; (b) by those 
already infected but in which the symptoms have not yet appeared: 
and (c) by the “carriers” of the virus, after recovery has taken place.: 
The pathogenic protozoa are transferred from infected to non- 
infected individuals by means of insects. They are carried from 
one locality to another in infected animals or intermediate hosts. 
Channels of infection. All infecting agents (bacteria, fungi and 
protozoa) must gain entrance to the tissues of the healthy animal 
before they can produce their respective disease. It is possible 
for a healthy animal to come-in close contact with a sick one and yet 
not become actually infected. Such an exposure does not neces- 
sarily mean that infection has occurred. Infecting agents gain acecss 
to the living tissues by being brought by some agency into the diges- 
tive, respiratory or generative tracts or by being introduced directly 
into the tissues. Most viruses are brought into the body with the 
food, infected dust-laden air, by ‘copulation or by inoculations. 
The common channels of infection, therefore, are: 
The digestive tract. Bacteria brought into the intestinal tract 
with food or water may pass through the mucosa of the intestine 
thereby gaining entrance into the walls of the intestine, or through 
the lymphatics or blood-vessels obtain passage to other organs or 
tissues. 
Respiratory tract. Infected, dust-laden air may be taken into 
the lungs where the infecting organism may either multiply or be 
taken by means of metastasis to other parts of the body. 
Generative tract. The viruses of certain diseases may be trans- 
mitted from one individual to another by copulation. 
By inoculation. Inoculation often takes place through abrasions 
or wounds produced accidentally on the skin or mucous membrane. 
In these cases the infecting organisms may be already lodged on 
the skin or membrane at the point of injury or they may be brought 
on the object causing the wound. 
