Anti-semitism is an ancient derangement, the oldest of hatreds, so it is strange that it lacks a more meaningful name. The misnomer “anti-Semitism” - a term coined in 1879 by the German agitator Wilhelm Marr, who wanted a scientific-sounding euphemism for Judenhass, or Jew-hatred - is particularly inane, since hostility to Jews has never had anything to do with Semites or being Semitic.

Perhaps there is no good name for a virus as mutable as anti-Semitism. “The Jews have been objects of hatred in pagan, religious, and secular societies,” write Joseph Telushkin and Dennis Prager in “Why the Jews?,” their classic study of anti-Semitism. “Fascists have accused them of being Communists, and Communists have branded them capitalists. Jews who live in non-Jewish societies have been accused of having dual loyalties, while Jews who live in the Jewish state have been condemned as ‘racists.’ Poor Jews are bullied, and rich Jews are resented. Jews have been branded as both rootless cosmopolitans and ethnic chauvinists. Jews who assimilate have been called a ‘fifth column,’ while those who stay together spark hatred for remaining separate.”

There was Jew-hatred before there was Christianity or Islam, before Nazism or Communism, before Zionism or the Middle East conflict. This week Jews celebrate the festival of Purim, gathering in synagogues to read the biblical book of Esther. Set in ancient Persia, it tells of Haman, a powerful royal adviser who is insulted when the Jewish sage Mordechai refuses to bow down to him. Haman resolves to wipe out the empire’s Jews and makes the case for genocide in an appeal to the king:

“There is a certain people scattered and dispersed among … all the provinces of your kingdom, and their laws are different from those of other peoples, and the king’s laws they do not keep, so it is of no benefit for the king to tolerate them. If it please the king, let it be written that they be destroyed.”

When the king agrees, Haman makes plans “to annihilate, to kill and destroy all the Jews, the young and the elderly, children and women, in one day … and to take their property for plunder.”

What drives such bloodlust? Haman’s indictment accuses the Jews of lacking national loyalty, of insinuating themselves throughout the empire, of flouting the king’s law. But the Jews of Persia had done nothing to justify Haman’s murderous anti-Semitism - just as Jews in later ages did nothing that justified their persecution under the Church or Islam, or their repression at the hands of Russian czars and Soviet commissars, or their slaughter by Nazi Germany. When the president of Iran today calls for the extirpation of the Jewish state, when firebombs are hurled at synagogues in London and Paris and Chicago, it is not because Jews deserve to be victimized.

Many Jews are no saints, but the paranoid frenzy that is anti-Semitism is not explained by what Jews do, but by what they are. They are the object of anti-Semitism, not its cause. That is why the haters’ rationales can be so wildly inconsistent and their agendas so contradictory. What do those who vilify Jews as greedy bankers have in common with those who revile them as fiendish Bolsheviks? Nothing, save an irrational obsession with Jews.

At one point, Haman lets the mask slip. He boasts to his friends and family of “the glory of his riches, and the great number of his sons, and everything in which the king had promoted him and elevated him.” Still, he seethes with rage and frustration: “Yet all this is worthless to me so long as I see Mordechai the Jew sitting at the king’s gate.” That is the unforgivable offense: “Mordechai the Jew” refuses to blend in, to be just like everyone else. He goes on sitting there - undigested, unassimilated, and therefore unbearable.

Of course Haman had his ostensible reasons for targeting Jews. So did Hitler and Arafat, so does Ahmadinejad. Sometimes the anti-Semite focuses on the Jew’s religion, sometimes on his laws and lifestyle, sometimes on his professional achievements. Under it all, however, it is the Jew’s Jewishness that the anti-Semite cannot abide.

With all their flaws and failings, the Jewish people endure, their role in history not yet finished. So the world’s oldest hatred endures too, as obsessive and indestructible - and deadly - as ever.