As a historian, I would call anti-Semitism an intellectual disease, a disease of the mind, extremely infectious and massively destructive. It is a disease to which both human individuals and entire human societies are prone. Anti-Semitism is very ancient, has never been associated with frontiers, and seems impervious to change. What strikes the historian is anti-Semitism’s fundamental irrationality. It seems to make no sense, any more than malaria or meningitis makes sense. In the whole of history, it is hard to point to a single occasion when a wave of anti-Semitism was provoked by a real Jewish threat (as opposed to an imaginary one).