A few years ago, a guest speaker made antisemitic statements on a panel at Princeton University, where I lead the Center for Jewish Life — Hillel. Alarmed by his comments that Israelis “are biped bloodhounds drinking the blood of one million [Palestinian] children,” I met with an administrator to discuss its impact on Jewish students. I was shocked to learn the administrator had never heard of the blood libel , the medieval lie accusing Jews of baking Passover matzo with the blood of Christian children. But why would he? He isn’t Jewish, and antisemitism hadn’t been a problem during his...