Book I CONTAGION 5

contagion; because contagion is a precisely similar infection of
the actual substance. Now when a house catches fire from the
burning of a neighboring house, are we to call that contagion?
No, certainly not, nor in general when the whole thing is de—
stroyed primarily as &a whole. 'The term is more correctly used
when infection originates in very small imperceptible particles,
and begins with them, as the word "infection' implies?; for we
use the term ^infected', not of a something that is destroyed as
a whole, but of a certain kind of destruction that affects its im—
perceptible particles. By the whole, I mean the actual com—
posite, and by very small, imperceptible particles, I mean the
particles of which the composite and mixture (combination)3
are composed. Now burning acts on the thing as a whole,
whereas contagion acts on the component particles, though
by them the whole thing itself may presently be corrupted and
destroyed.

Contagion, then, seems to be a certain passive affection of
elements in combination. But since such combinations can
be corrupted and destroyed in two ways, either by the advent
of a contrary element, owing to which the combination can—
not retain its form, or secondly by the dissolution of the com—
bination, as happens when things have putrefied, we may per—
haps hestitate to say whether contagion, when it is carried in
by infection of the smallest particles, is produced in the former
way or the latter. Moreover, what shall we say is the nature
of this infection?  Is it a corruption of those particles, or only
an alteration? What, in short, happens to those particles?
Hence it is hard to determine whether every contagion is &
kind of putrefaction. All these problems will become clearer
if we first investigate the fundamental differences of contagions,
and their causes. Meanwhile, if we allow ourselves to sketch
a sort of tentative definition of contagion, we shall define it as:
A certain precisely similar corruphton whch developes in the sub—
sfance of a combination, passes from one thing to another, and 1s
originally caused by infecthion of ihe «mperceptible particles.