Pasteur in his experiments noted that the bacterium responsible for fowl cholera or chicken cholera lost its virulence, i.e., the ability to cause disease after the bacterium was grown in the laboratory for long periods. He noted that the strains with decreased virulence were capable of inducing immunity against subsequent infections by their virulent counterparts. The Discovery of this phenomenon was a step towards successful inoculation with cowpox material against smallpox by Edward Jenner. Louis Pasteur used the term vaccine for the use of inoculation with avirulent (or nonvirulent) cultures to provide protection against disease.

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