Vertigo. And, that this circumstance exists in
respect to infectious matter appears from a known fact; that nurses, who
have had the small-pox, are liable to experience small ulcers on their arms
by the contact of variolous matter in lifting their patients; and that when
patients, who have formerly had the small-pox have been inoculated in the
arm, a phlegmon, or inflamed sore, has succeeded, but no subsequent fever.
Which shews, that the contagious matter of the small-pox has not lost its
power of stimulating the part it is applied to, but that the general system
is not affected in consequence. See Section XII. 7. 6. XIX. 9.

9. From the accounts of the plague, virulent catarrh, and putrid dysentery,
it seems uncertain, whether these diseases are experienced more than once;
but the venereal disease and itch are doubtless repeatedly infectious; and
as these diseases are never cured spontaneously, but require medicines,
which act without apparent operation, some have suspected, that the
contagious material produces similar matter rather by a chemical change of
the fluids, than by an animal process; and that the specific medicines
destroy their virus by chemically combining with it. This opinion is
successfully combated by Mr. Hunter, in his Treatise on Venereal Disease,
Part I. c. i.

But this opinion wants the support of analogy, as there is no known process
in animal bodies, which is purely chemical, not even digestion; nor can any
of these matters be produced by chemical processes. Add to this, that it is
probable, that the insects, observed in the pustules of the itch, and in
the stools of dysenteric patients, are the consequences, and not the causes
of these diseases. And that the specific medicines, which cure the itch and
lues venerea, as brimstone and mercury, act only by increasing the
absorption of the matter in the ulcuscles of those diseases, and thence
disposing them to heal; which would otherwise continue to spread.

Why the venereal disease, and itch, and tenia, or scald head, are
repeatedly contagious, while those contagions attended with fever can be
received but once, seems to depend on their being rather local diseases
than universal ones, and are hence not attended with fever, except the
purulent fever in their last stages, when the patient is destroyed by them.
On this account the whole of the system does not become habituated to these
morbid actions, so as to cease to be affected with sensation by