For more than a year the world has been on the lookout for any sign of avian flu spreading from birds to humans. The H5N1 virus has not yet mutated into a form easily transmissible from bird-to-man and thence from man-to-man, but because the devastation from a replay of an epidemic along the lines of 1918 would be so great, the vigilance and preparation has been high and sustained.

And this is a very, very wise course. One that should be emulated when it comes to the often equally deadly virus that transmits the most radical form of Islamist jihadism --the sort that triggers suicidal violence against everyone, including fellow Muslims, but most especially against America and Israel, their allies, and Jews everywhere.

It would be very useful if the world began to think of anti-Semitic rage as a virus that has sometimes consumed large parts of the globe and other times been suppressed to the point of invisibility. We have entered a period where the virus is replicating at an extraordinary rate, and where transmission is easy, and the appearance of the violent derangement that anti-Semitic Islamist extremism produces is both sudden and deadly.

In the years since 9/11, I count five discreet, possible instances of this virus suddenly appearing in the U.S., each of which resulted in death:

* The July 4, 2002 attack on the El Al counter at Los Angeles International airport;

* The October 1 University of Oklahoma suicide bombing outside of a crowded football game.

* The March 4, 2006 attack on the UNC campus

* The July 28, 2006 attack on Seattle's Jewish Community Center; and

* The August 29, 2006 attacks on pedestrians in San Francisco.

None of these five incidents is the sort of terrorist attack that we are most concerned about --the meticulously planned, coordinated attempt to cause spectacular devastation and casualties in the hundreds if not thousands. The London airlines bombing plot, like the London Tube attack, the Madrid rail attack, Bali resort bombing, and the Beslan school massacre are examples of this sort of terrorist atatck, and they rightly receive the bulk of Homeland Security's attention and analysis.

But isolated, single terrorist attacks are of great concern, and not just because of their real victims, but because of what they tell us about the spread of the hatred, and its potential to spread farther and faster than we ever dreamed.

It seems possible that we are close to a tipping point where so much anti-Semitic propaganda has been pumped into the world via new technologies and emboldened regimes and charismatic fanatics like Osama, Nasrallah and Ahmadinejad that the madness we would hope was confined to caves in Afghanistan and poor madrassas of Pakistan has in fact traveled far and at a furious pace, deep into France, now deep into London, and now we have to worry, deep into the United States.

It does not have to spread widely in American Muslim circles to provoke terrible violence by a few. There is no evidence of anything like the scale of Islamist hatred that consumes parts of France appearing in the U.S., but neither is it impossible that it will arrive here. What the five examples above underscore is that a segment of the Muslim population is susceptible to the deranging effects of anti-Western screeds from radical imams. That segment might be limited to the already deranged --the driver/killer in San Francisco and the shooter/killer in Seattle are both said to suffer from mental instability.

But the point is the segment exists, and individuals within it have killed.

What, exactly, triggered their violence?

I am keenly aware of the unfairness of taking the violence of extremist fanatics or the unstable and attributing it to a larger group to which the media assigns them. Thus Eric Rudolph, wrongly believed by many to be a Christian fundamentalist, became a whip with which to beat pro life Christians who reject violence at every turn even as they strenuously advocate for reductions in the number of abortions.

Responsible writers and analysts have to strain to repeat again and again that the vast, vast majority of Muslims in America, like the vast, vast majority of pro-life Christians in America, abhor and reject violence.

But responsible writers and analysts --and government officials-- also have to stand ready and ask, what are the sources of the anti-Semitic violence, just as they asked what were the sources of abortion clinic bombings or Timothy McVeigh's ravings or the Unabomber's craziness.

Is there a pattern, an epidemiology of this particular strain of violent hatred, and if so, how is it quarantined and combated?

Does the government have to more directly confront the sources of this hatred, whether in the Holocaust-denying and genocide threatening speeches of Ahmadinejad or the tapes that spring up from the caves of Pakistan?

And can we coherently argue that this evil ideology needs to be denounced by every person at every turn when we are extending visas to the former president of Iran so he can preach lies from the front of the National Cathedral?

Five years after 9/11 and there is an enormous amount of deep confusion in the country as to the threats we face, and the peril that these threats can cross oceans electronically and disturb and unhinge individuals.

We are rightly spending hundreds of millions on early detection and treatment systems for a virus that does not yet exist.

We need to do the same for a virus that not only exists but which has already killed in America.