Deborah Lipstadt has twice been famous, once for a lawsuit, now for a movie, each time because of the mission to which she's devoted her life:

Combating those who deny that the Holocaust took place, that the Nazis systematically exterminated six million European Jews in the 1940s.

It sounds ludicrous. Deny the Holocaust? That's like claiming the Earth is flat.

But the phenomena is real - and insidious, experts say, particularly in a modern "soft core" approach that skirts outright denial but distorts established fact in a way that subverts historical truth.

On Sunday, Lipstadt will deliver the keynote speech at a Holocaust teach-in at Gratz College, where high demand caused the institution to expand and then close registration for the event.

More than 700 people, many of them teachers, are coming to the Cheltenham campus from as far as Missouri and Maine, converging at a moment when science and fact are often dismissed as unreliable or ambiguous.

Fake news floods the internet. On Tuesday the nation elected Donald Trump as president, choosing a candidate who began his political career by spreading the false claim that President Obama was not born in this country.

"Given all the lies and misstatements of facts that he made, and that seemed not to have made a difference to people," Lipstadt, 69, said in an interview, "it is further proof that, though we may think deniers are akin to flat earthers, there will be people who believe."

Lipstadt, a professor of modern Jewish history at Emory University in Atlanta, stands among the nation's top authorities on denial and anti-Semitism.

She earned her reputation the hard way.

In 1996 author David Irving sued Lipstadt for libel in England - where such cases put the burden of proof on the defendant - for calling him a denier in her book Denying the Holocaust: The Growing Assault on Truth and Memory.

The court ruled against Irving, saying he had deliberately misrepresented and manipulated historical evidence - specifically, that the Holocaust occurred.

The new British-American screen drama Denial, starring Rachel Weisz as Lipstadt, tells the story of the court case.

The movie tagline: "The whole world knows the Holocaust happened. Now she needs to prove it."

That's not as easy as it sounds. Today the internet offers easy distribution for disinformative stories.

Websites such as Real Jew News and Bible Believers aim to expose the Holocaust as a hoax, while YouTube videos purport to reveal "the truth" about the mass killings.

"I don't believe the official version of the Holocaust is true, whatsoever," Pastor Steven Anderson of Faithful World Baptist Church in Arizona says in a video that's been viewed 380,000 times. "There are lots of things about the story that don't add up."

Others believe that too, despite statements by authorities including the American Historical Association, which asserts: "No serious historian questions that the Holocaust took place."

Yet a 2014 study by the Anti-Defamation League found that 35 percent of the world population never heard of the Holocaust. Among those who had, 32 percent believed it was either a myth or greatly exaggerated.

That, Gratz organizers said, explains the importance of the teach-in, which also will examine genocides like the one that occurred in Darfur during the 2000s.

"There's this question: Does the Holocaust conceal other genocides, because it takes up so much room, or does it reveal other genocides?" asked Rutgers University doctoral candidate Jeff Benvenuto. "I think it's both."

At Gratz, Benvenuto teaches classes on the genocide carried out against American Indians, and how those killings have been subsumed by a larger, celebratory story of winning the West.

In fact, he said, nations often deny genocides that occurred at their founding, not wanting to cloud heroic creation stories. For instance, the Ottoman government exterminated about 1.5 million Armenians in 1915. Today the government of Turkey, its successor state, denies that genocide occurred.

Holocaust denial goes as far back as 1942, when the Germans sought to hide their annihilation of the Jews, according to the U.S. Holocaust Memorial Museum, and it persists. This year lawmakers in Poland sought to ban phrases like "Polish concentration camps," saying that implied guilt for Nazi death camps operated in occupied Poland.

"It's not all about saying it never happened at all, it's about minimizing it," said Heidi Omlor, a Maine high school social-studies teacher who will be at Gratz on Sunday. "It takes away from those who died. It takes away from the survivors. It takes away from their testimony."

She teaches her students: Be alert to official-looking websites that promote falsehood as truth, or those that arrange facts in a way that's deceptive.

For example, she said, deniers often claim the Auschwitz gas chambers did not function long enough to kill six million people. And that's true. But it ignores the fact that millions were killed elsewhere, in other countries by different methods.

The thing about genocide, she said, is it starts small, with insults and petty cruelties that seem inconsequential.

"I tell the kids it starts in the lunchroom," Omlor said. "Find the kid that sits alone in the lunchroom. Go sit with him. Find someone who has been bullied. Stand up for them."

Her students study the "Pyramid of Hate," which shows how bias can grow in complexity, from a base of insensitive jokes to a pinnacle of mass murder.

The answer?

"In a day and age of so much open prejudice and hatred," Lipstadt said, "it behooves good people, when someone says something hateful and prejudicial: Say something. Do something."