3. In fact, inoculation is sometimes known to fail, whatever method is used; nor do I think this will happen oftener by this than by any other. We see many patients inoculated with fresh matter, in the usual and strongest manner, yet without any eruption, who afterwards have the small-pox, either in the natural way, or upon a second trial. We likewise daily see people, often and long exposed to the infection, thinking themselves safe, who have afterwards catched the natural disorder. It is certain, there are some who never have it; whole families are free from it for many generations; and it has been observed, that upon a hundred persons dying of old age, five or six had escaped it, though equally exposed with their contemporaries. Inoculators have met with much the same proportion of fruitless attempts. A person in this case can never have an absolute certainty of being for ever safe, but only a probability proportionable to the number of experiments, the goodness of the matter, &c.
It certainly is a desideratum, to be able constantly to communicate the small-pox if the subject is capable of receiving it; or to know, in case of failure, where the fault lies. It is to be hoped this problem will one day be solved, when all disputes about the expediency of inoculation are at an end, and we fix our whole attention on the improvement of it.
In order to attain this desirable end, I would recommend the following rules: