Question

1. what observation made edward jenner think that if a person is exposed to a germ, their immune system will “know” how to fight it the next time it encounters the same germ?

Explanation:

Edward Jenner noticed milkmaids who had contracted cow - pox, a milder disease, were immune to smallpox, a more severe one. This led him to the idea of immunity.

Answer:

Milkmaids with cow - pox were immune to smallpox.