Book I CONTAGION 55

hand, if it does occur, whence does it arise, seeing that the
same causes that were in the first body are not present in the
second? For in the first there were as causes, obstructions,
plethora and malignity of the humors, and the like; but, in '
the second, none of these are necessarily present, for we observe
that a person of moderate habit and in almost perfect health3*
may nevertheless catch contagion from another, the germs
alone being sufficient to convey it. We must say, then, that
in the second individual, also, foul and confined putrefaction
takes place, foul because there is deep—seated corruption of what
receives putrefaction from the germs, on account of the strong
adhesion and activity of these germs, and also on account of
their analogy; confined too is this sort of putrefaetion because
of the great evaporation that takes place; but even if it were
not of such a sort per se, that matters not, provided it is deep—
seated. For precisely the same sort of principle and the same
sort of germ has arisen in the second body as in that first af—
fected; since, as we have said, the germs have in themselves
the power to propagate and engender what is similar to them—
selves, just as the spirits do. Now in the first body were pres—
ent those causes which are wont to produce putrefactions every—
where in our bodies, I mean obstructions, plethora and un—
sound condition of the humors, from which causes foul, con—
fined putrefaction mainly results; thence are born germs that
are suited to convey the contagion to another, whether there
were or were not in that other the same causes and disposi—
tions as existed in the first. Since they have encountered an
analogous humor, they transfer the contagion to a second and
third individual, and so to others also.

"That the principles and germs of contagions can also reach
us from without, and may not occur in us originally, is equally
manifest. For we often observe diseases roving about and
affecting many people, and one calls them Epidemics. / Of these
some are common to many communities or districts, but are
not contagiocus, and they are called merely 'common'. But
others again are contagious, that is when once they are en—
gendered in an individual, and from the general disposition of
the atmosphere, they transfer the contagion to another, and
these are called not merely 'common', but contagious as well.
Of this sort are pestilences such as that plague about which
"TThueydides!' writes, which roved all over Greece, and like