CHAPTER XII
OTHER DIFFERENTIAE OF CONTAGIONS

We have still to investigate other characteristic differences
of contagions and their causes. For all contagions do not be—
have in one and the same manner. Some arise first in ussös
and are engendered in one part of the body, and presently pass
thence into another part. Others reach us first from without,
and, when developed, are propagated from one part of the
body to another. Some rove over the periphery and barely
attack the outer skin, others seize the more solid parts, others
again the internal organs, others all parts of the body, whether
external or internal. Some again take hold and infect very
quickly, others more slowly; some manifest themselves at once,
others not till long after the infection; certain of them are fatal,
certain others do not endanger life. But all those that have
their birth in a live body disseminate infection there, though
not in a dead body. And some bodies catch infections very
easily, others either not at all or with difficulty. Hence we may
ask whether we can immunize ourselves against pestilences as
we can against poisone.

Now it is clear that the germs of contagions arise originally
in ourselves, not only in the case of scabies, achores and phthisis,
but also in the case of fevers that are called pestiferous.  'The
reason is that in us and our humors there is nothing to prevent
the production of putrefactions that are both foul and con—
fined, from which develop germs that are both viscous and
have a strong combination. I have said above that it is these
germs that convey contagions, and no one can doubt that they
may be produced thus originally in some individual. But per—
haps it may be doubted whether in the second individual also, ;
when affected, there occurs likewise a foul and confined putre—
faction, or not. For if that putrefaction does not occur, how
are there produced in the second individual new germs which
have the power to infect some third individual? On the other

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