CHAPTER X

WHV SOMEH DISEASES ARE CONTAGIOUS, OTHERS NOT, AND WHT
MILD DISEASES ARE CONTAGIOUS

First let us consider why some diseases are, and others are
not, contagious, and how it happens that some, though they
are more inflammatory and more dangerous, are nevertheless
not contagious, whereas others that run a milder and gentler
course are highly contagious  This is a difficult question; for
if contagion follows on force and activity, then it would seem
that the more acute diseases should be the more contagious.
And if contagion is the result of adustion, as many doctors
think, then it would appear, likewise, that those which are
more inflammatory ought to be more contagious. Again, if
contagion is the result of great putrefaction, how is it that in
many eases there is great putrefaction but no contagion? Now
no diseases that occur without putrefaction are ever contagious,
because, as has been said, without some sort of putrefaction
there can be no contagion.  Moreover, even in cases where
there is putrefaction, I maintain that keenness also is needed,
if contagion is to follow, but keenness and power to become
active are not enough, for viscosity also is needed, and, as I
have said, a strong and thoroughly worked combination.  Like—
wise, contagion is not the result of adustion, but if in con—
tagious diseases there is also adustion, this is rather the
result than the source of the contagion. Again, contagion
does not result from every sort of putrefaction, however exten—
sive that putrefaction may be, and affecting a great mass of
humor. No, it results only from that putrefaction in which
germs that have a strong combination and are constituted in
viscosity can come into being. Among fevers there are many
that are very inflammatory, for instance that which is due to
bile, but they have a dry composition, and the particles that
evaporate from them cannot be the germs of contagion in some—
thing else, whether because their combination is feeble, or be—

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