16 CLASSIFICATION OF DISEASES 
other with the degree of virulence of the infecting microdrganism. 
This has been expressed in the formula 
pe 
R 
in which D = the disease, V= the virulence of the infecting organism 
and R = the resistance of the host or the individual attacked. 
As V or R change the disease is modified. For example, rabbits 
that are partially immunized against swine plague bacteria, when 
inoculated with a virulent culture of that organism, will live for 
several days and perhaps weeks, and then die of peritonitis, pleuritis 
or extensive pus formations, instead of perishing within twenty- 
four hours with bacteriemia as they would if they had not been par- 
tially protected against this organism. In chronic cases of swine 
plague, as found in certain outbreaks, the bacteria are often atten- 
uated so that when inoculated into susceptible rabbits the result 
is the same as when the rabbits protected by partial immunization 
were inoculated with virulent cultures. The above simple formula 
which was worked out and demonstrated for certain swine diseases 
seems to apply to infectious diseases generally. 
Classification or grouping of the infectious diseases. It will 
be found in the study of the morbid anatomy of the various specific 
maladies that the lesions in a given disease vary in different species 
and to a marked degree in individuals of the same species. This 
fact precludes the possibility of classifying or arranging them after 
their morbid anatomy. If the infectious diseases are to be considered 
as parasitisms, as they appear to be, the logical method of classify- 
ing them for purposes of study would seem to be the one suggested 
by their etiology, namely, that they shall be placed in groups cor- 
responding to the classification of their etiological factors. Thus 
a single lesion found in the glands of the head, in the lungs, in the 
liver, in the mesenteric glands, in the skin, joints, generative organs 
or elsewhere in the body would be called tuberculous if the bacterium 
of tuberculosis could be demonstrated to be its cause. The same 
conclusion would be maintained regardless of the character of the 
lesion, whether it consisted of a clump of epitheliod cells, purulent, 
caseous or calcareous tissue. These facts are enough to indicate 
that the most direct method of arranging these diseases for purposes 
of study is in groups composed of like generic etiological factors. 
