“Why?”
The answer to the question is so important that at the risk of repetition I give it explicitly.
The Inquiry was suppressed because of its ascription of cowpox to horsegrease.
It was the belief of dairy-maids in Gloucestershire that to have had cowpox was to be secure from smallpox. Jenner was much impressed by the rustic superstition, and brought it so persistently before his medical brethren at their convivial assemblies, that they threatened to expel him if he bored them any longer with the subject. “It is true,” they said, “that the maids believe an attack of cowpox keeps off smallpox, but we know they are wrong; for we are all familiar with cases of smallpox after cowpox.”
Thus frustrated, Jenner’s ingenuity took another turn. It was the belief of farriers that if infected in dressing horses’ greasy heels, they too were secure from smallpox. The area of this conviction was narrower than that of the dairy-maids, farriers being neither so numerous, nor so observant of their beauty: but Jenner entertained their faith and converted it to his purpose.
Horsegrease protected from smallpox, if cowpox did not. But might there not be one sort of cowpox that answered to the dairy-maids’ faith, if another sort did not? Happy thought! The defensive sort was derived from the horsegrease which protected the farriers: the non-defensive originated spontaneously on the cows. Men, fresh from handling horses’ greasy heels, milked cows and communicated to them the horses’ disease. Milkmaids, who in turn contracted from the cows that sort of pox, were like the farriers secure from smallpox, yea securer; whilst milkmaids who contracted pox spontaneously developed on cows were not secure. The milkmaids’ superstition was therefore justifiable: they were right and they were wrong—right when they got pox through the cow from the horse; wrong when they got pox from the cow simply.
Why then, it may be asked, did not Jenner dismiss the cow from consideration? Why did he not base his prescription on the farriers’ experience, and use and recommend horsegrease exclusively for inoculation? The question is an obvious one, but it is not easy to make out Jenner’s answer with precision. His assertion was, that—
The virus from the horse is not to be relied upon as rendering the system secure from variolous infection, but the matter produced by it on the nipple of the cow is perfectly so—
which was to say that horsegrease attained its highest prophylaxy after transmission through the cow.