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 observed that milkmaids who had contracted cowpox (a mild, similar disease) did not get smallpox, a deadly virus. He inferred their immune systems learned to fight the related germ, protecting them from the more severe illness.

Answer:

Edward Jenner observed that milkmaids who had recovered from cowpox (a mild viral infection) did not develop smallpox, a much more dangerous and often fatal disease. This led him to hypothesize that exposure to a less harmful germ (cowpox) allowed the immune system to recognize and fight off the similar, more deadly germ (smallpox) later.