se two diseases for a time led Jenner to question
the possibility of doing so. After careful investigations, however, he
reached the conclusion that there was a difference in the effects of the
two diseases, only one of which produced immunity from small-pox.

"There is a disease to which the horse, from his state of domestication,
is frequently subject," wrote Jenner, in his famous paper on
vaccination. "The farriers call it the grease. It is an inflammation and
swelling in the heel, accompanied at its commencement with small cracks
or fissures, from which issues a limpid fluid possessing properties of a
very peculiar kind. This fluid seems capable of generating a disease
in the human body (after it has undergone the modification I shall
presently speak of) which bears so strong a resemblance to small-pox
that I think it highly probable it may be the source of that disease.

"In this dairy country a great number of cows are kept, and the office
of milking is performed indiscriminately by men and maid servants. One
of the former having been appointed to apply dressings to the heels of
a horse affected with the malady I have mentioned, and not paying due
attention to cleanliness, incautiously bears his part in milking the
cows with some particles of the infectious matter adhering to his
fingers. When this is the case it frequently happens that a disease is
communicated to the cows, and from the cows to the dairy-maids, which
spreads through the farm until most of the cattle and domestics feel its
unpleasant consequences. This disease has obtained the name of Cow-Pox.
It appears on the nipples of the cows in the form of irregular pustules.
At their first appearance they are commonly of a palish blue, or rather
of a color somewhat approaching to livid, and are surrounded by an
inflammation. These pustules, unless a timely remedy be applied,
frequently degenerate into phagedenic ulcers, which prove extremely
troublesome. The animals become indisposed, and the secretion of milk is
much lessened. Inflamed spots now begin to appear on different parts
of the hands of the domestics employed in milking, and sometimes on the
wrists, which run on to suppuration, first assuming the appearance of
the small vesications produced by a burn. Most commonly they appear
about the joints of the fingers and at their extremities; but whatever
parts are affected, if the situation will admit the superficial
suppurations put on a circular form wi