Those who cannot remember the past are condemned to repeat it. (George Santayana)

The episode is one of the Colonial America's most notorious cases of mass hysteria. It has been used in political rhetoric and popular literature as a vivid cautionary tale about the dangers of isolationism, religious extremism, false accusations, and lapses in due process.

It was not unique, but a Colonial American example of the much broader phenomenon of witch trials in the early modern period, which took place also in Europe. Many historians consider the lasting effects of the trials to have been highly influential in subsequent United States history. According to historian George Lincoln Burr, "the Salem witchcraft was the rock on which the theocracy shattered."

McCarthyism is the practice of making accusations of subversion or treason without proper regard for evidence.

It also means "the practice of making unfair allegations or using unfair investigative techniques, especially in order to restrict dissent or political criticism." The term has its origins in the period in the United States known as the Second Red Scare, lasting roughly from 1950 to 1956 and characterized by heightened political repression against supposed communists, as well as a campaign spreading fear of their influence on American institutions and of espionage by Soviet agents. Originally coined to criticize the anti-communist pursuits of Republican U.S. Senator Joseph McCarthy of Wisconsin, "McCarthyism" soon took on a broader meaning, describing the excesses of similar efforts.

The term is also now used more generally to describe reckless, unsubstantiated accusations, as well as demagogic attacks on the character or patriotism of political adversaries.

No victim has come forward. There's no investigation. And physical evidence? That doesn't exist either.

But thousands of people are convinced that a paedophilia ring involving people at the highest levels of the Democratic Party is operating out of a Washington pizza restaurant.

The story riveted fringes of Twitter - nearly a million messages were sent last month using the term "pizzagate".
So how did this fake story take hold amongst alt-right Trump supporters and other Hillary Clinton opponents?

Let's start with the facts.

In early November, as Wikileaks steadily released piles of emails from Clinton's campaign chairman John Podesta, one contact caught the attention of prankster sites and people on the paranoid fringes.

James Alefantis is the owner of Comet Ping Pong, a pizza restaurant in Washington. He's also a big Democratic Party supporter and raised money for both Barack Obama and Hillary Clinton. He was once in a relationship with David Brock, an influential liberal operative.

Alefantis - who's never met Clinton - appeared in the Podesta emails in connection with the fundraisers.

And from these thin threads, an enormous trove of conspiracy fiction was spun.

Users of 4chan, a message board known for free speech, extreme content and trollish behaviour, began posting speculation and supposed connections gleaned from internet searches. They trawled Alefantis' Instagram feed for pictures of children and the modern art which lines his restaurant's walls, and dreamt up a paedophile sex ring involving prominent politicians and political donors.

Soon there were protesters outside of Comet Ping Pong. Alefantis even invited some of them in - they filmed the encounter and put it on YouTube.

"They ignore basic truths," Alefantis tells BBC Trending. For instance, the conspiracy supposedly is run out of the restaurant's basement. "We don't even have a basement."

"Sometimes an innocent picture of a child in a basket is just an innocent picture of a child in a basket and not proof of a child sex trafficking ring," he says.

The conspiracy theory bubbled up from 4chan onto the mainstream internet when a Reddit user posted a long document with all of the "evidence" several days before the US election. It first appeared on a section of the site popular with Donald Trump supporters from the extremist white nationalist alt-right.