Shortly before undergoing surgery so that he could donate bone marrow to a 14-year-old boy in France, Derrick Nelson, a high school principal in Westfield, N.J., granted an interview to a student journalist. “If it’s just a little bit of pain for a little bit of time that can give someone years of joy,” he told her, “it’s all worth it.” In October, an organization that connects people suffering from life-threatening diseases with potential bone marrow donors had contacted the principal after he showed up as a potential match for the French patient. When follow-up testing confirmed the match, the...