Have you heard about the areas of Europe, or perhaps even of the United States, that are run by jihadists and which non-Muslims can't even enter?

Don't get too worried if you haven't: They don't exist. Or maybe you have, if you watch Fox News or read snippets of Louisiana Governor Bobby Jindal's speech about Islamic extremism. While politics is rife with falsehoods, myths, and baseless rumors, it's often tough to see exactly where a claim comes from, and how it reaches a wider audience—including, for example, a Republican governor with apparent presidential aspirations and a reputation as a sober policy wonk. Here, however, it's possible to follow forensic traces and see how some small elements of truth metastasized into an outlandish claim.

Here's what Jindal said, via CNN:

In the West, non-assimilationist Muslims establish enclaves and carry out as much of Sharia law as they can without regard for the laws of the democratic countries which provided them a new home ....

It is startling to think that any country would allow, even unofficially, for a so called "no-go zone." The idea that a free country would allow for specific areas of its country to operate in an autonomous way that is not free and is in direct opposition to its laws is hard to fathom.

When CNN reporter Max Foster pressed him on this passage after the speech, Jindal couldn't name any specific instances. But he pointed to a report in the famously unreliable tabloid Daily Mail, and couched his refusal to back down as bold truth-telling: "I think that the radical Left absolutely wants to pretend like this problem is not here. Pretending it's not here won't make it go away."

Jindal's claim was particularly brazen because it came just a day after Fox News apologized for a comment an alleged expert made on air. Steve Emerson, who analyzes terrorism for the network, said on Fox News Sunday, "There are actual cities like Birmingham that are totally Muslim, where non-Muslims just simply don’t go in." British Prime Minister David Cameron ripped Emerson as only an old Etonian can: "When I heard this, frankly, I choked on my porridge and I thought it must be April Fools' Day. This guy’s clearly a complete idiot." Emerson apologized, but other Fox personalities repeated variations on the theme, and eventually Fox issued on-air corrections on both Fox & Friends and Justice With Judge Jeanne.

Fox isn't known for shying away from controversial claims, so the decision to move so aggressively to reverse is an indicator of how wrong the claim is. Of course, that came after several days of Fox personalities repeating the claim casually on air. (The retraction didn't come in time for one group, apparently: Paris' mayor said Tuesday that Paris would sue Fox News for sullying its image.)

But where did the claim come from in the first place? Like many political myths, there's a partial basis in fact that has become exaggerated into a hyperbolic and, in this case, inflammatory and dangerous claim.

It seems to stem from two or maybe three real phenomena. The first is the presence of sharia courts in some places in Europe. In the United Kingdom, for example, "Muslim Arbitration Tribunals" are officially mandated but set up outside the court system and can resolve civil and family issues through Islamic law; there are also reports of informal religious courts. There are similar Jewish courts in Britain, and the Muslim tribunals have received encouragement from figures including then-Archbishop of Canterbury Rowan Williams. On the other hand, there are convincing arguments that the courts can sometimes be bad for women. (There's a fascinating echo of the Ottoman empire's "millet" system, in which non-Muslims were allowed to set up their own courts to deal with matters of personal law.)

The second real phenomena is the rise of vigilante sharia squads in some places. For example, in Whitechapel, East London, CNN reported on bands of Muslim men who try to keep alcohol out of the area and harangue passers-by about morality. RedState's Erick Erickson thinks he's caught CNN red-handed: While the network criticizes Jindal for not knowing of any real no-go zones, CNN itself reported on one! But the analogy doesn't quite hold. What's happening here is disturbing, but well-short of extremist-run enclaves. These are just ad-hoc groups, and area Muslims by and large condemn it in CNN's reporting. There's no evidence that these squads are powerful or widespread.

The third factor is what are known as "Zones Urbaines Sensibles," or "sensitive urban zones," in France. These areas are defined by their socioeconomic status—they're characterized by high unemployment, high rates of public housing, and low educational attainment. As it happens, many of these areas are populated largely by poor immigrants from the Muslim world, creating a neat but misleading correlation. Some of the "no-go" coverage has suggested that police and other emergency services dare not go into these areas. The United States is sadly not immune to dangerous city areas where emergency-service providers feel unsafe, so in that way this is a universal phenomenon. But as BusinessWeek notes, it's not the case that the government has written these zones off; in fact, they've been designated for further attention and work on urban renewal.

These factors seem to have combined to create the no-go-zone myth. Daniel Pipes, an often-inflammatory critic of Islam, seems to be patient zero for the meme. In 2006, he coined the phrase "no-go zones" to describe the SUZs, but after visiting in 2013, Pipes revised his views:

For a visiting American, these areas are very mild, even dull. We who know the Bronx and Detroit expect urban hell in Europe too, but there things look fine. The immigrant areas are hardly beautiful, but buildings are intact, greenery abounds, and order prevails.

These are not full-fledged no-go zones but, as the French nomenclature accurately indicates, "sensitive urban zones." In normal times, they are unthreatening, routine places. But they do unpredictably erupt, with car burnings, attacks on representatives of the state (including police), and riots.

Having this first-hand experience, I regret having called these areas no-go zones.

Yet contacted by BusinessWeek last week, he again affirmed that there are no-go zones in Europe. I've asked Pipes to explain his re-reversal and haven't yet heard back, but I'll update if I do.

If you dig through the fever swamps of the Internet, you can see the idea spreading since then. For example, the blog "Violence Against Whites" chronicles the SUZs and other alleged no-go zones across Western Europe. (Other material on the site includes claims that Boers were ethnically cleansed in South Africa and a section on "white martyrs.") Catholic.org reports, "These areas are Muslim-dominated neighborhoods. Non-Muslims dare not set foot into the areas."

Meanwhile, the meme can be seen extending to the United States. Truth Uncensored reports, incorrectly, that there are no-go zones stateside, including in places like Dearborn, Michigan, a Detroit suburb with a large Muslim population. Conservative Tribune even posts a map that allegedly shows no-go zones controlled by Islamists across the United States. I can't tell where the map originally came from, but it cites data from Steven Emerson, the Fox expert who apologized for his no-go-zone comments. And the map is posted elsewhere on the Internet, labeled as everything from a map of terrorist camps (apparently al-Qaeda is big in Boca Raton—alert your grandparents!) to areas with concentrated Muslim populations.

Bottom line: You don't need to worry about Muslim no-go zones if you live in the United States. And if you're planning a tourist expedition to Europe, it's a good idea to avoid high-crime areas, regardless of their demographics. But why, if there's no evidence for no-go zones and some of the highest-profile propagators of the idea have repudiated it, do such myths survive and thrive?

It probably has a lot to do with the conservative media ecosystem. Erroneous beliefs such as these tend to concentrate along people's partisan or ideological axes. (The same is true of liberal media, though not in this particular case.) And once an idea has taken seed, it's extremely difficult to root out. As political scientists Brendan Nyhan and Jason Reifler have shown, corrections can actually backfire, increasing holders' faith in their incorrect beliefs.

Unfortunately, even reporting on these misconceptions can worsen the problem, so I am part of the problem. But it seems important to note that Jindal is plainly wrong. These sorts of distortions and exaggerations don't help to fight the very real threat of Islamist terror. They don't serve the cause of creating an informed, reasoned democratic society. And they don't help the political prospects of guys like Jindal, who has previously demanded that his GOP stop being "the stupid party." Maybe this meme is the real no-go zone.