WASHINGTON -- It so happened that this week, on the day I wrote about Holocaust denial in the Middle East, a homegrown denier took a rifle into the United States Holocaust Memorial Museum -- an institution where I sit on the governing board. The museum counts about 1.7 million visitors each year who learn about the history of murderous racism -- and now one who decided to add to that history.

That day, out of curiosity, I did something I rarely do. I read the Internet comments on my column from different sources. In addition to the normal political vituperation, the level of anti-Jewish feeling was appalling. The European genocide, some contended, was exaggerated by Jews for political purposes. Jews were behind the Bolshevik Revolution, the rise of Hitler and the outbreak of World War II. They control the newspapers, radio, television and book publishing. Mahmoud Ahmadinejad is right to expose the Holocaust myth, and Israel is perpetrating the real holocaust against the Palestinians.

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Of course, these are the views of a small, self-selected group of the unbalanced -- hundreds out of millions. But the Internet allows these obsessions to gather in fetid pools, as James W. von Brunn (a prolific Web author) knew and exploited. The Internet has helped to create communities of malice.

The anti-Semitic community is varied in background and ideology. It includes both Internet Nazis and campus leftists carrying signs that read, "Jews (equals) Nazis." The Rev. Jeremiah Wright recently blamed "them Jews" for blocking his access to President Obama. A conservative Web site recently included a forum on Holocaust denial (before it was exposed and removed). One posting read: "The same blinded people that believe that the Germans intentionally killed Jews -- also believe the myth of the Anne Frank Diary."

Marginalized Western anti-Semites and Holocaust deniers have gained influence in unexpected places. In 2002, Libya's dictator Moammar Gaddafi awarded his (less than coveted) Gaddafi International Human Rights prize to Roger Garaudy, a French Holocaust skeptic. Ahmadinejad's 2006 conference of deniers featured David Duke as a keynote speaker.

For some Americans, anti-Semitic conspiracy theories are akin to a shameful hobby -- like collecting old racist knickknacks or Nazi memorabilia. But these ideas are not harmless, because they can inspire an angry, obsessed bigot who sets out on a June morning to kill Jews -- and murders an African-American man who had a wife and young son.

The durability of anti-Semitism is a horrifying marvel of history. Sara Bloomfield, the director of the Holocaust Museum, observes: "Anti-Semitism has existed with and without Christianity. With and without the right wing. With and without the left wing. With and without democracy. With and without economic problems. With and without globalization. With and without a Jewish homeland."

Why the Jews? It is a question that must often have been asked during pogroms and in ghettos and in prison camps. There are many answers, and none of them adequate. Anti-Semitism in the West has undeniable theological roots -- the distortion of a faith, founded by a Jewish teacher, to justify the persecution of Jews. Anti-Semitism has been fed by government incitement and by blood libels that never seem to die. It found resonance in various forms of nationalism and nativism, in the bent science of eugenics, and eventually in totalitarian ideology.

David Berger, the editor of "History and Hate," writes, "We shall never fully understand anti-Semitism. Deep-rooted, complex, endlessly persistent, constantly changing yet remaining the same, it is a phenomenon that stands at the intersection of history, sociology, economics, political science, religion and psychology."

But we do know that anti-Semitism has always been a kind of test -- a reliable measure of a nation's moral and social health. When the rights of Jews are violated, all human rights are insecure. When Jews and Jewish institutions are targeted, all minorities have reason for fear. And by this standard, America has cause for introspection.

The museum that von Brunn assaulted is the best answer to his hatred -- the aging survivors who still volunteer, photographs revealing the vanished lives of the dead, the happy pictures drawn by murdered children. Not far from where von Brunn entered the museum, there is a black wall inscribed with a quotation: "All men are created equal ... they are endowed by their creator with certain inalienable rights ... among these are life, liberty and the pursuit of happiness."

This is what anti-Semitism ultimately must deny; and this is the reason anti-Semitism must always and everywhere be confronted.