CHAPTER V
CONTAGION äT A DISTANCE

Evyven more surprising and hard to explain are those diseases
that cause contagion, not by direct contact only, or by fomes
only, but also at a distance. 'There is a kind of ophthalmia
with which the sufferer infects everyone who looks at him. It
is well known that pestiferous fevers, phthisis and many other
diseases infect those who live with the sufferer, even though
there is no actual contact. It is far from certain what is the
nature of these diseases, and how the taint is propagated.  We
must therefore study these problems with the greatest care,
since of this sort are the majority of the diseases that we are
investigating. 'This sort of contagion seems to have a dif—
ferent nature from the others and to work by means of a dif—
ferent principle. In the first place, because there are certain
pestiferous fevers which are fatal within ten or twelve hours,
though the patient feels neither heat nor chill. 2 Likewise,
when a person with ophthalmia gives the disease to another,
the infection again seems to have a different nature, since visual
sensation is not produced by heat or cold, but is due to the so
called manifestations and images of objects.8 Furthermore,
the sudden and almost instantaneous penetration of these con—
tagions is another proof of this difference, for on a sudden, and
by a blow, so to speak, of the eye, they penetrate through the
whole individual and destroy it. Now there is no one of the
known qualities that can do this so promptly, because they have
an opposite to contend with. Moreover, if this contagion were
produced by known qualities, it would always be propagated
in what is weaker and has less resistance. 'This is evidently
not the case, for the weaker organ often suffers little or not at
all.  For instance, phthisis does not attack the eyes, though
they are tender and delicate, but seizes the lungs. Finally,
since this contagion is carried from all sides and to every part,
it seems to imitate the motion of spiritual qualities, which

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