Question: In the 18th century, smallpox was a common and deadly disease. A country doctor, Edward Jenner, observed that patients working with cattle who contracted a mild disease called cowpox, never came down with the more serious smallpox. When Jenner obtained liquid from cowpox sores and injected it into healthy patients, they became immune to smallpox. What term best describes Jenner's treatment for smallpox?

  Options: 
 A. vaccination
 B. gene therapy
 C. chemotherapy
 D. genetic engineering
Answer: A