About the matter of this prophylactic there was from the first a curious confusion which continues to this day.
Jenner was a country doctor at Berkeley in Gloucestershire, a dairy country, where the maids believed that if they caught cowpox in milking they could never afterwards catch smallpox. Jenner when a young man was inclined to accept the dairymaids’ faith; but when he discussed it with his medical acquaintance, they ridiculed him. They said, “We know that such is the dairymaids’ faith, but we also know that it is untrue; for we know dairymaids who have had cowpox, and afterwards had smallpox notwithstanding their cowpox.” Jenner was convinced and said no more about cowpox.
To this point let me draw special attention. No man knew better than Jenner that cowpox as cowpox was no preventive of smallpox.
Toward middle-life he had what he conceived to be a happy thought. Cowpox as cowpox he had dismissed as impracticable; but there was a variety of cowpox which he resolved to recommend.
Cows in Gloucestershire were milked by men as well as by women; and men would sometimes milk cows with hands foul from dressing the heels of horses afflicted with what was called grease. With this grease they infected the cows, and the pox which followed was pronounced by Jenner to have all the virtue against smallpox which the dairymaids claimed for cowpox.
HORSEGREASE COWPOX.
According to Jenner, then, the dairymaids were right, and they were wrong. They were right when the pox they caught was derived from the horse through the cow; they were wrong when the pox they caught originated on the cow without the horse. He thus discriminated a double pox—cowpox of no efficacy against smallpox, and horsegrease cowpox of sure efficacy.
Further, in this connection, it is to be observed, that farriers believed that when they got poisoned in handling horses with greasy heels, they too, like the dairymaids, were safe from smallpox.
It is not therefore for cowpox, but for horsegrease cowpox that Jenner is answerable. In cowpox he had not, and could have no faith.
In 1798 Jenner published his famous Inquiry, a treatise much more spoken of than read, wherein he distinctly set forth the origin of his chosen prophylactic. It was not, I repeat, cowpox: it was horsegrease cowpox. He carefully discriminated it from spontaneous cowpox, which, he said, had no protective virtue, being attended with no inflammation and erysipelas, the essential sequences of inoculation with effective virus.