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nicated to the latter by cohabitation. There is, consequently, no doubt abou^ 
swine-plague being a communicable disease—one that can be transmitted by 
inoculation and by cohabitation. 
Its relative distribution and prevalence, as compared with hog cholera, are 
questions that remain to be answered. If the lesions were always so character¬ 
istic that a diagnosis could be made by an ordinary post-mortem examination, it 
would be an easy thing to put a force of men in the field and determine them 
in a single season, but so long as a bacteriological study must be made of each 
case it, of course, requires a much longer time to obtain satisfactory results. 
My first impression was that swine-plague was of much less importance from 
an economical point of view than hog cholera, but our recent investigations 
have modified these views, and have indicated that the former disease may be 
as prevalent and cause as heavy losses as the latter. 
The great aim in bacteriological investigation, as in all other branches of 
medical science, is to discover the means of preventing or curing diseases. The 
discovery of the microbe, and the study of its life history, are but steps in the 
accomplishment of this object. We have, therefore, from the first endeavored 
to bring out such facts as would enable us to form a science for the prevention 
of germ diseases. 
It was once my good fortune to listen to a series of lectures by that great 
naturalist, Louis Agassiz, and I was deeply impressed with a statement of his, 
that Nature held her secrets with so tight a grasp that an investigator might 
consider his life successful if during the whole course of it he had been able to 
discover and contribute to science but a single new principle. He, himself, after 
a life of almost unparalleled activity in scientific research, laid stress upon but 
one such discovery, although he had contributed an enormous mass of facts. In 
our investigations we have discovered two such principles, which have been 
and are destined to be of very great value to sanitary science. 
The first of these principles is that with nearly all diseases the most suscep¬ 
tible animals have a certain power of resisting germs of the greatest virulence. 
Or, putting the same idea in other words, we may say that, by diminishing the 
dose of virus, we reach a point where a considerable number of germs may be 
introduced into the tissues without being able to cause disease. The second 
great principle is that with non-recurrent diseases the germs produce during 
their cultivation outside of the animal body certain chemical substances which, 
when administered to susceptible animals, grant immunity from the effects of 
that germ. These principles, discovered and demonstrated in the course of the 
investigations under our Department of Agriculture, promise to be of more value 
to sanitary science than all the other principles which the combined workers of 
the world have contributed to bacteriology since Koch demonstrated the germ 
theory of disease. 
The easy, safe, and certain production of immunity in animals exposed to 
contagion would be a most powerful weapon for the control of germ diseases, 
and, consequently, our investigations have been for years turned in this direc¬ 
tion. We have tried the Pasteur method of vaccination, using attenuated virus 
or vaccine ; we have tried inoculation on the principle discovered by us of pro¬ 
ducing the desired effect by means of graduated doses of strong virus, and we 
