Which best explains why the dairymaids were immune to smallpox?

Jenner, a physician and scientist, noticed that milkmaids generally didn't develop smallpox, a disfiguring and sometimes deadly disease. He guessed it was because they sometimes caught cowpox, a related disease that only caused mild illness in people.

To understand what really happened, Boylston, whose research interest is the history of smallpox inoculation, explored letters, medical notes and research papers, going back to about 1720. He has self-published a book about it, Defying Providence: Smallpox and the Forgotten 18th Century Medical Revolution. At that time, he says, doctors were attempting to prevent smallpox through a process called variolation.

Explanation:

This was all that I could find...sorry :)

-NPR

can you give me more to work here with plzs and you