IT was due to certain particularly favorable circumstances that the first ideas on preventive inoculation were gathered from observations on smallpox patients. Such circumstances were presumably the following:
a. It is a disease which attacks epidemically, in a short time and within a small area, large numbers of people, thus permitting of easy comparisons and suggesting conclusions from the facts observed.
b. Its fatality is comparatively small, so that after each outbreak a large number of convalescent persons remain alive to serve as objects for future observation and comparison.
c. These convalescents are marked and are thus easily distinguishable from the rest of the population who have not been attacked, and even the severity of the disease they have gone through is, so to say, written down on their faces and bodies.
d. The disease is easily communicable, owing to the infectious matter appearing on the surface of the patient's body in the pustules.
It was easy, therefore, to notice in this case, as was indeed very early done in the East, that a person who has gone through one attack, as shown by his pitted face, very rarely suffers even during severe subsequent epidemics. Smallpox, like other epidemic diseases, breaks out in some years in very fatal, in others in milder forms. It is admissible that by a mixed process of thought and faith an impression insensibly gained ground that it was lucky to have been touched by the smallpox deity—of course, not in years when that deity appeared in terrifying mortality. Accordingly, in times of mild outbreaks people would not be