Answer: Cowpox

There is an old English saying “smooth as a milk maid’s skin” that relates the smoothness of the thing in question to the smoothness of the skin of a milk maid. To a modern listener, it could easily be written off as some curious but antiquated saying. On reflection, perhaps it refers to the creamy white skin of milk maids depicted in pastoral paintings from the 18th century. Maybe exposure to the cream in whole milk made their hands really soft. Maybe milk maids used moisturizer to keep the udders of the dairy cows supple and, in the process, made their own hands soft as a result.

The origin of the phrase is far more complex and interesting than such simple explanations would provide for, however. Milk maids, through their close work with cows, were exposed to the cowpox virus. Exposure to the cowpox virus imparted a partial immunity to the far more deadly and disfiguring smallpox virus. As such, milk maids would have skin unblemished by the scarring of smallpox infection and they’d have very smooth skin compared to the other people in their villages.

While that alone is an interesting little historical tidbit, what’s even more interesting is that the milk maids’ seeming immunity to smallpox was pivotal in directing 18th century scientists to study the connection and eventually develop a method of using cowpox to vaccinate people against smallpox.