Which of the following best defines variolation as it was historically practiced for smallpox?

• A. Deliberate inoculation of healthy people with material from smallpox lesions to induce immunity
• B. Injection of a cowpox extract that contains only isolated proteins to elicit a focused immune response while avoiding transfer of full infectious virus particles
• C. Administration of a chemically weakened smallpox virus produced in laboratory cell culture under sterile conditions to generate immunity without disease
• D. Genetically engineering a related but nonvirulent virus to express smallpox antigens and then giving it to people to trigger long-lasting protective immunity

Answer: (A)