BOOK I
CONTAGION

CHAPTER I
WHAT IS CONTAGION?

I shall now proceed to discuss Contagion, and shall begin with
what seem to be its universal principles from which are derived
its particular causes. Many of the questions investigated in
my treatise 'On the Sympathy and Antipathy of Things',: were
there discussed because of their connection with this subject.
As its name indicates, contagion is an infection that passes
from one thing to another. For, to produce contagion, two
factors must always be present, though they may be either two
different things or two continuous parts of thesame thing. How—
ever, the term is naturally and properly used only when the
two things in which it is produced are separate. When it is
conveyed from one part to another of the same thing, it is not
contagion, properly speaking, but only a sort of contagion.
The infection is precisely similar in both the carrier and the
receiver of the contagion; we say that contagion has occurred
when a certain similar taint has affected them both.  So, when
persons die of drinking poison, we say perhaps that they were
infected, but not that they suffered contagion; and in the case
of things that naturally go bad when exposed to the air, such
as milk, meat, etc., we say that they have become corrupt,
but not that they have suffered contagion, unless indeed the air
itself has also become corrupt in a precisely similar way. In
what follows I shall investigate this question more carefully.

Everything that happens, whether actively or passively, af—
fects either the essential substance of bodies or their non—essen—
tial parts. When someone has been heated or sullied by some—
thing, we do not say, except by a metaphor, that he has suffered

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