with the
results. The tests were conducted in such a way as to preclude the
possibility of error. It was shown, in a word, that by the simple
process of inoculating well animals with the modified poison the
infectious disease might be avoided.

It were long to tell the story of the experimentation and discovery
that now followed. The last quarter of the century has been fruitful
in the greatest results. The bacilli of one disease after another have
been discovered, and the means have been invented of defending the
larger animal life from the ravages of microscopic organisms.

But what is an "attenuated" virus? Pasteur and other scientists have
shown that by the inoculation of suitable material, such as a piece of
flesh, with the poison of a given disease, the bacteria on which that
disease depends rapidly multiply and diffuse themselves through the
substance. If poison be taken from the _first_ body of infected
material and carried to _another_, that other becomes infected; and
from that the third; from the third the fourth, and so on to the tenth
generation.

It was noticed, however, that with each transference of the virus to a
new organic body the bacilli were modified somewhat in form and
activity. They became, so to speak, less savage. The bacterium which
at the beginning had been for its savagery a wolf, became in the
second body a cur; then a hound; then a spaniel; and then a
diminutive lapdog! The bacteria were thus said to be "domesticated;"
for the process was similar to the domestication of wild animals into
tame. The virus was said to be "attenuated;" that is, made thin or
fine; that is, its poisonous and death-dealing quality, was so reduced
as to make it comparatively innocuous.

If after the process of attenuation was complete--if after the
bacteria were once thoroughly domesticated and the poison produced by
them be then introduced into a well subject, that subject would indeed
become diseased, but so mildly diseased as scarcely to be diseased at
all. In such a case the result was of a kind to be called in popular
language a mere "touch" of the disease. In such case the severe
ravages of the malady would be prevented; but the subject would be
rendered incapable of taking the disease a second time.

On this line of fact and theory Pasteur successfully pressed his work.
One disease after another was investigated. It was demonstrated in the
case of both the lower animals and men that a large number o