CHAPTER III.
IMMUNITY AND vSUSCKITIIHUTY.

ONE of the most interesting1 things observed in physi-
ology and pathology is the resistance which certain ani-
mals show to the invasion of their bodies by the germs
of disease.

Tims, man suffers from typhoid fever, cholera, and
other infectious diseases which are never observed in the
domestic animals; cattle are subject to a pleuro-pneumo-
nia which does not affect their attendants; man, the cow,
and the guinea-pig are peculiarly susceptible to tubercu-
losis, which the cat, dog, and horse resist; yellow fever
is a highly contagious, infectious disease which is almost
certain to attack all new arrivals of the human species
when epidemic, but which rarely, if ever, attacks animals.

The popular mind accepts the statement of such facts
as these without any other explanation than that the
animals are different, and so of course their diseases are
different; but the more the scientific man contemplates
them, the more complicated the matter becomes; for,
while it might be admitted that a difference in the body-
temperature and chemistry might explain why a frog
will resist anthrax, which readily kills a white mouse, it
will not explain why a house-mouse, whose chemistry
must be almost identical with that of the white mouse>
can successfully combat the disease. Nor is this all.
That one attack of yellow fever, of typhoid fever, or
of scarlet fever renders a second attack almost impos-
sible is not the less interesting because of its every-clay
observation. The mouse that has recovered from teta-
nus will not take tetanus again, and most interesting and

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