monstration of the fly's
agency in the transmission of malignant pustule and typhoid fever, and
that of certain mosquitoes in the conveyance of yellow fever and
malarial disease. We now know that bad air (the original meaning of the
word _malaria_) has nothing to do with fever and ague, and that swamps
are not unwholesome if they are free from infected mosquitoes. The
mosquito does not originate the malarial infection; it simply serves as
the temporary host of the micro-organism (_Plasmodium malarioe_) which
is the cause of the disease, having obtained its transient "guest" from
some human being. Consequently, marshy districts that are full of
mosquitoes are not malarious unless the mosquitoes are of the kinds
capable of lodging the plasmodium, and unless there is or has recently
been present in the neighborhood some person affected with malarial
disease. Moreover, the most virulently malarious region is a safe place
of residence for human beings, provided they protect themselves
absolutely against the bite of the mosquito. This has been strikingly
demonstrated in the case of the Roman Campagna.

From the disease-producing animal parasites we come now to those that
are believed to be of vegetable nature. Under the general name of
_bacteria_, there are multitudes of micro-organisms having pathogenic
powers, each giving rise to some definite specific disease, and certain
associations of different bacteria causing particular morbid conditions.
Generations ago physicians had a glimmering of what we now term the germ
theory of disease, as was shown by their use of such expressions as
_materies morbi_ and morbid poisons. Even the definite relationship of
special microscopic organisms to individual diseases was foreshadowed by
Salisbury nearly fifty years ago. But it was not until years after those
conceptions, and in no wise descended from or led up to by them, that an
intelligible and satisfactory germ theory of disease was formulated.

It is to Pasteur, the immortal chemist, that we owe this theory, as well
as that of the attenuation of viruses--both of more than theoretical
import, since they have given us aseptic surgery, the power of
frequently preventing hydrophobia, the antitoxine treatment of
diphtheria, and the ability to stay the hand of Death in the form of
many a stalking pestilence. Every infectious disease is now held to be
due to its own particular micro-organism, and many diseases that were
not until recently