There is a disease to which the Horse, from his state of domestication, is frequently subject. The Farriers have termed it THE GREASE. It is an inflammation and swelling in the heel, accompanied in its commencement with small cracks or fissures, from which issues a limpid fluid, possessing properties of a 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 the Smallpox, 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 THE COWPOX. Thus the disease makes its progress from the Horse (as I conceive) to the nipples of the Cow, and from the Cow to the Human Subject. (Pp. 2 and 6.) This conception of the origin and progress of the disease was not Jenner’s specially: he shared it with the farmers to whom it was a novelty-- The rise of Cowpox in this country may not have been of very remote date, as the practice of milking Cows might formerly have been in the hands of women only; which I believe is the case now in some other dairy countries; and consequently that the Cows might not in former times have been exposed to the contagious matter brought by the men servants from the heels of Horses. _Indeed a knowledge of the source of infection is new in the minds of most of the farmers in this neighbourhood_, but has at length produced good consequences; and it seems probable from the precautions they are disposed to adopt, that the appearance of the Cowpox here may either be entirely extinguished or become extremely rare. (P. 56.) Thus Cowpox was to be extinguished by forbidding milkers to handle Horses’ greasy heels. Jenner himself tried to produce Cowpox in the manner described, but without success-- It is very easy [he wrote] to procure pus from old sores on the heels of Horses. This I have often inserted into scratches made with a lancet on the sound nipples of Cows, and have seen no other effects from it than simple inflammation. (P. 45.)