Who used observational data in developing the concept of vaccination?

• A. James Lind
• B. Pierre Louis
• C. John Snow
• D. Edward Jenner

Answer: (D)

Observational evidence of natural immunity sparked the idea of vaccination. Edward Jenner noticed that dairy workers who contracted cowpox seemed protected from smallpox. This pattern suggested that a prior, milder infection could confer immunity to a more dangerous one. Jenner then tested the idea by deliberately introducing cowpox material to a healthy child and later showing protection when exposed to smallpox. This use of real-world observations to infer a protective effect, followed by a deliberate test of that inference, is what established the vaccination concept.

Other figures contributed important methods in medical history, but Jenner is the one whose work directly linked a naturally occurring, milder infection to protection against a more dangerous disease, giving birth to vaccination.