I enjoyed this article in a recent issue of Commentary by Paul Johnson, one of my favorite modern historians.
Just to give you a taste:
But if anti-Semitism is a variety of racism, it is a most peculiar variety, with many unique characteristics. In my view as a historian, it is so peculiar that it deserves to be placed in a quite different category. I would call it 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.
What strikes the historian surveying anti-Semitism worldwide over more than two millennia is its 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). In Japan, anti-Semitism was and remains common even though there has never been a Jewish community there of any size.
Like many physical diseases, anti-Semitism is highly infectious, and can become endemic in certain localities and societies. Though a disease of the mind, it is by no means confined to weak, feeble, or commonplace intellects; as history sadly records, its carriers have included men and women of otherwise powerful and subtle thoughts. Like all mental diseases, it is damaging to reason, and sometimes fatal.
The piece is a fairly long one, but one that, at least in my opinion, is well worth reading in its entirety.