Question

Edward Jenner’s smallpox inoculation experiment was based on his observation that some dairymaids who worked with cows had suffered lesions on their hands, but that these dairymaids were immune to the more severe smallpox. Which best explains why the dairymaids were immune to smallpox?
They did not interact with many people so they were not exposed to the pathogen.
They frequently washed their hands so they were able to wash away the pathogen.
They were exposed to cowpox, and the substance their body produced to fight cowpox also attacked smallpox.
They breathed the air around the cows that carried an antipathogen in it that destroyed the pathogen.

Answer 1

They were exposed to cowpox, and the immune response their bodies made to cowpox also protected against smallpox.

Brief explanation: cowpox and smallpox are closely related orthopoxviruses, so antibodies and memory immune cells generated against cowpox cross-reacted with smallpox, providing immunity—this is the basis of Jenner’s vaccination.