DISSEMINATION OF VIRUS 13 
is also true that there is a striking similarity between the symptoms 
exhibited in many diseases. The principle was laid down by French* 
“that a particular symptom attracts special notice in a given case, 
and that the diagnosis has to be established by differentiating be- 
tween the various diseases to which this symptom may be due.” 
As a rule it is difficult to make a positive diagnosis of a specific, 
infectious disease from the symptoms alone. 
Tissue changes. - The changes produced in the tissues by a specific 
microorganism are usually constant in their nature. They represent 
the result of the reaction of the body tissues to the particular invading 
virus. In the invasion of the animal body certain viruses seem 
to select some definite tissue or organs in which to locate. On the 
other hand, the location of the lesions due to other organisms is largely 
determined by the channel through which they gained entrance 
and the place where they first became lodged. The diagnostic 
value of the specific lesions, or other tissue changes, therefore, may 
be relied upon in a large number of cases with greater certainty than 
the symptoms. In some diseases, like rabies, the characteristic 
tissue changes are microscopic and cannot be determined from the 
gross examination of the organs. It is very important in making 
a diagnosis from the lesions, that the specific tissue changes be pres- 
ent and that they be carefully differentiated from secondary ones 
that often occur in such diseases. 
In making a diagnosis from the morbid tissues it is necessary 
that sufficient time should have elapsed for the changes to have taken 
place. If the microdrganism should be very virulent, death may 
take place before the usual changes have occurred. Likewise, if 
an animal is killed in the earlier stages of the disease, the characteris- 
tic lesions are very liable not to be found because they have not as 
yet had time to develop. 
The specific cause. The positive diagnosis is made when the micro- 
érganism that caused the disease is demonstrated in the morbid 
tissues. In some diseases this can be done by very simple bacteriolo- 
gical methods. With other affections it requires very careful and 
special methods to isolate, cultivate or even to demonstrate the cause. 
There are several epizodtic diseases for which a specific organism 
has not as yet been found. In order to use the cause, for diagnostic 
*Index of Differential Diagnosis of Main Symptoms by various writers edited by 
Herbert French, M.A., M.D., Wm. Wood & Co., New York, 1913. 
