Which of the Following Findings Was Essential for Edward Jenner's

Multiple Choice

Which of the following findings was essential for Edward Jenner's vaccination process?

A) A weakened microorganism will not cause disease.
B) Pathogenic microorganisms infect all humans and animals in the same manner.
C) Someone who recovers from a disease will not acquire that disease again.
D) Exposure to a milder disease form may produce immunity.
E) Disease is caused by viruses.