Vaccination for the prevention of smallpox was the first indication that an animal disease could supervene a human disease; that one disease could modify or prevent another, and that a great plague could possibly be abolished from the earth.
It is not our plan just now to outline, even in brief, the marvelous historical record, or the clinical facts as to it for the last hundred years, but only to specify and briefly note changes of view that have modified what we may call the Jenner impression as to it.
1. We have probably come to know that more than one animal disease has the power by inoculation of modifying smallpox. Jenner thought that it was the disease known as "grease" in horses, modified by passage through the cow, and again in man so as to prevent smallpox. Now it is probable that there are two or three forms of