The discovery of vaccines stands as one of the most significant breakthroughs in the history of medicine. The technique of inoculation, which works on the same principle as vaccines, had been known for cen- turies before Dr. Edward Jenner developed the first vaccine in the 1790s. But inoculation was a haphazard, risky treatment. Those who practiced it did not under- stand why it worked when it did, and sometimes it did not work. In any case, nearly everyone who was inoculated against smallpox had to endure at least a mild attack of the disease before they could acquire immuni- ty from it.

On the other hand, Jenner's vaccine and those later developed by his medical successors offered the advantage of providing immunity to a disease with extremely low chance of contracting the disease and showing any symptoms. What...