4. Which of the following is an important characteristic of a scientific theory? a, proven by more than one scientist
b. unchanged by new evidence
c. accepted by the public
d. supported by much evidence

In 1775 Jenner began to study the relationship between cowpox (a comparatively harmless disease) and the more dangerous and disfiguring smallpox. ... He hypothesized that exposure to cowpox rendered the body immune from smallpox. After nearly twenty years of experiments, he developed the first vaccine.