When the word hero is mentioned, people usually envision a mighty warrior vanquishing an enemy or a firefighter rescuing children from a burning building. Though these examples of courage and ability certainly qualify as heroes, so do some scientists and doctors searching quietly and patiently, day after day, year after year, for new ways to combat disease. One of the greatest heroes of modern times was such a person. In the 1790s, a mild-mannered English country doctor named Edward Jenner discovered that he could prevent a dreaded disease by injecting patients with tiny samples of a similar but less deadly disease. He called his discovery a vaccine.

A Trail of Misery and Death

The formerly terrifying disease from which Jenner and his successors rescued millions of people was smallpox. Its characteristic symptoms...