Antisemitism is a virus that needs a host to survive.
In the ancient world the host was the Imperial rule of the Roman Empire. Jews were strange because of their customs and their monotheism. The belief in one invisible God amounted to atheism in a polytheistic world. Jews were persecuted in the same way Christians were in ancient Rome.
In medieval Europe the host was fear. Europeans were afraid of the plague and needed a scapegoat to blame. The church prohibition of Christians from loaning money created a niche market for Jewish bankers. Antisemitism grew in this fertile soil of epidemic and economic fear.
In the Reformation, Martin Luther didn't help the situation by reminding the world that Jews were involved in Jesus' murder. And wouldn't you know, the new host became Christianity itself.
This, of course, led to antisemitism finding one of its favorite hosts in the German National Socialist movement. Nazism was born.
In post-WWII America, this favored host got a face lift. The virus of antisemitism found a host that was uniquely American. Freedom of Speech became a petree dish for this social contagion. This gave birth to the Neo-Nazi movement.
And now we see yet another host for this virus. Extremists in Fundamentalist Islam breed antisemitism better and faster than post-Katrina Golf coast homes breed mold. This host organism of terrorism is the latest in a long line of hosts who have caught this illness.
Every stage in history has seen a new host spreading this ancient virus. And every stage has also seen a new antiretrovirual drug which tries to stamp out its nemesis. But like all viruses, antisemitism keeps evolving, morphing and multiplying.