All of a sudden nostalgia for 1930s Germany resurges across Europe, where some dismiss it as “just fun.”

Carnival marchers in the Belgian town of Aalst last week were dressed up as Jewish caricatures, as if lifted from the pages of the crude Nazi-era weekly Der Sturmer. They had long, crooked noses, Hasidic hats, rat tails — the works. Men clad in SS uniforms marched alongside, drinking beer and joking with the cheering crowds.

The grotesque display, which appeared in Aalst’s annual three-day carnival last year for the first time and repeated again last week, was described to the BBC by the town mayor’s spokesman as “our humor, just fun.”

Men in Nazi uniforms and women in concentration camp pajamas also marched last week in another European town, Campo de Criptana in Central Spain. Oh, and music bands performed in full Nazi regalia in Croatia, Ukraine and elsewhere.

All in good fun, no doubt.

Meanwhile, a study conducted among 14,000 people in 16 European Union countries found that a quarter of all responders oppose having “too many Jews” in their country. The study, presented Monday to the European Jewish Association’s annual conference in Paris, also found 21 percent believe Jews “talk too much about the Holocaust” and exploit it for their own purposes.

There’s “freedom of expression” in Belgium, the country’s UN ambassador, Marc Pecsteen de-Buytswerve, told me Wednesday. On the other hand, he added, the country has a “legal framework that prevents racism, anti-Semitism and discrimination.” The Belgian justice department is reportedly investigating the Aalst case, and the courts will weigh both factors, he said.

Meanwhile, the United Nations Security Council didn’t discuss European anti-Semitism during the month of February, when Belgium served as its rotating president. The topic is not on the council’s agenda. Israel is, on the other hand, and in the monthly session dedicated to the Israeli-Palestinian dispute, European members denounced its policies in highly moralistic tones.

Ah, but haven’t you heard? Criticizing Israel has nothing to do with anti-Semitism. And yet, in the study presented in Paris, 25 percent said that when they think of Israel’s politics, they understand why some people hate Jews. And a similar percentage equated Israelis to Nazis in their behavior toward the Palestinians.

Such views aren’t held by far-right haters alone, but also by Europe’s leftists and in Muslim immigrant circles.

Britain’s far-left Labour Party leader, Jeremy Corbyn, lost an election in December by a wide margin, partly because many Brits — not just the Jews who traditionally vote Labour but spurned it in droves this time — believed his heated anti-Israel rhetoric crossed over into virulent anti-Semitism.

Yet, America’s current leader among Democrat presidential candidates, Bernie Sanders, flew over to campaign for Corbyn. Sanders surrounds himself with characters like Palestinian activist Linda Sarsour and Rep. Ilhan Omar, seen by many Americans as anti-Semitic. They too say they only criticize the policies of Benjamin Netanyahu, the elected Israeli prime minister that Sanders calls a “reactionary racist.”

Sanders says he’s “proud to be Jewish,” with many relatives who perished in the Holocaust. His followers will gladly (and rightly) denounce Europe’s Nazi nostalgia and right-wing bigotry. But, as in the past, when it was embraced by both Bolsheviks and tsarists alike, today’s anti-Semitism defies both the politics of the left and the right.

“It’s a virus that has no antidote or vaccine, so it flourishes in many environments,” says Abraham Foxman, former head of the Anti Defamation League. The virus, he says, was suppressed after WWII, but now, in a post-truth Internet era, it’s no longer taboo. So as it raises its head again, he says, “we have to build new firewalls.”

Because if the past can teach us anything, Europe’s newly discovered “fun” will end in global tears.