Jenner's discovery that cowpox provides immunity to smallpox.

• A. Cowpox provides immunity to smallpox
• B. Smallpox causes cowpox
• C. Cowpox has no relation to smallpox
• D. Milkmaids are immune to all diseases

Answer: (A)

The main idea here is that Jenner showed a milder, related infection could protect against a deadlier one. Cowpox infection trains the immune system in a way that also guards against smallpox, because the two viruses share similar features that the body recognizes. This is the basis of vaccination: using a harmless or less harmful organism to provoke immunity to a more dangerous one.

Historically, milkmaids who caught cowpox seemed immune to smallpox. Jenner demonstrated this by infecting someone with cowpox and later exposing them to smallpox, after which the person did not develop smallpox. This provided practical proof that vaccination could prevent a dangerous disease.

So the statement that cowpox provides immunity to smallpox best captures Jenner’s finding: a safer, related infection can confer protection against a more dangerous disease. The other ideas don’t fit: smallpox does not cause cowpox; cowpox and smallpox are related, not unrelated; and milkmaids’ protection isn’t immunity to all diseases, just to smallpox in this context.