Francis Folger Franklin

In 1736, four-year-old Francis contracted the smallpox virus and died shortly thereafter.
Benjamin Franklin, who had been inoculated earlier in his own life, had intended for his son to be inoculated as well. However, due to an illness affecting Francis at the time planned for his inoculation, the procedure was postponed.
His death devastated both his parents, who doted upon Francis, and after this incident, Franklin became "the most eloquent advocate of smallpox inoculation." Provided by Wikipedia