(CNN) Earlier this week, a Russian Orthodox cleric investigating the 1918 assassination of Czar Nicholas II and his family raised the possibility that it was a " ritual murder. "

To many observers, that statement might sound strange, but more or less inoffensive. To Jews, however, it raised the haunting specter of "blood libel," a pernicious and long-lasting lie about Jews murdering Christian children and using their blood in religious rituals.

"The ritual murder charge, coming from a Russian Orthodox priest, gives undeserved credence to a core anti-Semitism myth long propagated by fringe elements in Russian society," said David Harris, CEO of the American Jewish Committee, "and, more generally, fits into the incendiary conspiracy theories connecting Jews to every calamity known to humankind."

Used for hundreds of years as a pretext to torture, imprison and kill Jews, the "blood libel" myth may be the worst religious lie in circulation, but it is far from the only one

The Russian "ritual" charge came the same week that Pope Francis visited Myanmar, a country where Buddhist monks have spread lies about the persecuted Muslim Rohingyas. It also came the same week that President Donald Trump shared three anti-Muslim videos on Twitter. Originally posted by a far-right group in Britain, the videos purported to show Muslims committing violence against innocent youths and smashing a Christian statue.

As my colleagues have pointed out, Trump has a history of telling untruths about Muslims , from the fiction that some in New Jersey celebrated the 9/11 attacks to the idea that American mosques are hotbeds of radicalism. Both falsehoods continue to spread despite consistent debunking.

I wanted to understand why religious lies are so dangerous and difficult to eradicate, so I called up E.M. Rose, a visiting fellow at Harvard University and author of the book, " The Murder of William of Norwich: The Origins of the Blood Libel in Medieval Europe.

Rose's book explodes one of the most common myths about "blood libel" -- that it was primarily perpetrated by angry mobs reacting against economic stresses and widespread social change.

Instead, Rose argues that the anti-Semitic slanders spread because powerful people wanted them to -- an idea that may hold lessons in our age of "fake news" and fast-twitch social media.

Here's a transcript of my conversation with Rose, which has been lightly edited for clarity and length.

Q: Had you heard about the new "ritual murder" accusations in Russia?

E.M. Rose: No, but it does not surprise me.

Q: Why not?

Rose: Because a lie, when repeated over and over by increasingly eminent people, takes on a life of its own. What causes people to start the lie may be different from the motivations of those who repeat it, but it eventually becomes a narrative embedded in culture. You see it take on local color, and thus the aura of fact, but the characters and broad outline get passed on from generation to generation.

You tell a very simple lie with a very coherent narrative, and it can reverberate for centuries and do real harm. Initially, people say, "This is nuts, and it will pass." But it doesn't. That's what's so scary.

Q: Let's talk about how the "blood libel" started. I've read that scholars don't really know.

Rose: My argument is that I can show exactly when it started. It was the Middle Ages, and a knight came back to England after the Second Crusade, which was a total disaster. The knight was a complete screw-up and had borrowed a lot of money to go on the crusade. The knight was deeply in debt to his banker, who happened to be Jewish, so he had him killed in order to avoid having to pay up.

When the knight goes on trial everyone knows he's guilty, but his lawyer still has to come up with a defense. His defense is to blame the victim. He says you can't convict the knight for killing the banker until you convict the banker and his fellow Jews for killing a child who had died a few years before. The story raised enough doubts that the knight got off the hook.

Q: How does it spread from there?

Rose: It gets picked up later by the monks of Norwich, who are trying to make a saint of William of Norwich, the child supposedly killed by the banker and fellow Jews, and unite the country after a long civil war. So they blame a third party, outsiders, the Jews. From there, it is used in England and France and elsewhere to extort Jews, increase feudal lords' power and make themselves look good.

Q: Tell me more about the context of these lies. Was it a time of cultural, economic and political upheaval?

Rose: When I first started looking at "blood libel," I thought that might be the case. That the Jews were largely an immigrant community, and there was social upheaval and economic stagnation. But none of that is true about the 12th century. Actually, it was a fairly boom time, economically. And Jews were certainly a marginal group, but they were not necessarily all that vulnerable.

The point is, as "blood libel" spread, they became more and more vulnerable. At first, nobody believed it. It was not credible. But as it gets told over and over, as people tell the story in different ways, more and more people believe it.

Q: Did they really believe it, though, or were they using "blood libel" to get what they wanted, even though they knew it was false?

Rose: They would often say things like, "Well, I know that this case is false, but there's another one that's true." In other words, they may deny a specific incident while insisting that the myth is still largely true.

I guess what struck me in my research was the instrumentalism of the lie -- it isn't mob ignorance and anger; it is purposeful, told by people in power to get more power.

Q: What was it about Jewish Europeans in the Middle Ages that made them so vulnerable to "blood libel." That they were considered "the other"?

Rose: Actually, the threat was not that they were not integrating. It was that they were integrating. They were maintaining their social ties while becoming more and more a part of the larger communities in which they lived. So then you start to see lies about secret rituals and how we should all be aware of their "true" purposes for being in the country. You see the same thing happening now with Muslims in America, where people in power raise concerns about Sharia and paint all Muslims as terrorists.