Owing to the
introduction of regulations which were based on the knowledge of the
cause of the disease and the life history of the organism, together
with the prophylactic inoculation devised by Pasteur, the incidence of
the disease has been very greatly lessened. Looking at the matter from
the lowest point of view, the money which has been saved by the
control of the disease, as shown in its decline, has been many times
the cost of all the work of the investigations which made the control
possible. It is a greater satisfaction to know that many human lives
have been saved, and that small farmers and shepherds have been the
chief sharers in the economic benefits. The indirect benefits,
however, which have resulted from the application of the knowledge of
this disease, and the methods of investigation developed here, to the
study of the infections more peculiar to man, are very much greater.

FOOTNOTE:
[1] The interesting analogy between fermentation and infectious
disease did not escape attention. A clear fluid containing in solution
sugar and other constituents necessary for the life of the yeast cells
will remain clear provided all living things within it have been
destroyed and those in the air prevented from entering. If it be
inoculated with a minute fragment of yeast culture containing a few
yeast cells, for a time no change takes place; but gradually the fluid
becomes cloudy, bubbles of gas appear in it and its taste changes.
Finally it again becomes clear, a sediment forms at the bottom, and on
re-inoculating it with yeast culture no fermentation takes place. The
analogy is obvious, the fluid in the first instance corresponds with
an individual susceptible to the disease, the inoculated yeast to the
contagion from a case of transmissible disease, the fermentation to
the illness with fever, etc., which constitutes the disease, the
returning clearness of the fluid to the recovery, and like the
fermenting fluid the individual is not susceptible to a new attack of
the disease. It will be observed that during the process both the
yeast and the material which produced the disease have enormously
increased. Fermentation of immense quantities of fluid could be
produced by the sediment of yeast cells at the bottom of the vessel
and a single case of smallpox would be capable of infecting
multitudes.

CHAPTER VI

CLASSIFICATION OF THE ORGANISMS WHICH CAUSE DISEASE.--BACTERIA: SIZE,
SHAPE, STRUCTURE, CAPACITY FO