Question

what was jenners observation?

Explanation:

Identify Jenner's historical context

Edward Jenner was an English physician. He studied smallpox prevention.

Analyze the key observation

Jenner observed local dairymaids. They contracted mild cowpox.

Connect cowpox to smallpox

Dairymaids with cowpox avoided smallpox. They showed immunity.

Formulate the scientific conclusion

Cowpox exposure protects against smallpox. This led to vaccination.

Answer:

Edward Jenner observed that milkmaids who had contracted cowpox (a mild disease caught from cows) were immune to smallpox (a deadly disease).