Edward Jenner discovered vaccinations. He found that some of the dairymaids who worked with cows had suffered lesions on their hands, and 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.

The dairymaids possessed immunity against the more extreme smallpox as due to getting exposed to cowpox, the body of the dairymaids generated certain kinds of substance to defend against the cowpox, which also helped in defending smallpox.  

When Edward Jenner found that the dairymaids suffering from cowpox possessed immunity towards smallpox, he concluded that the emergence of cowpox at the same time developed immunity towards smallpox. To ensure, Jenner collected a fluid sample from the pustule of cowpox and administered it within a boy's body, and thus, discovered the principle of immunity.  

They were exposed to cowpox, and the substance their body produced to fight cowpox also attacked smallpox.

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