The proliferation of fake news online has roiled the American presidential election, politicians around the world and internet companies including Facebook and Google.

So what happens when the fake news juggernaut also sweeps up ordinary citizens?

Cecilia Kang saw the fallout up close when she interviewed James Alefantis, owner of the Comet Ping Pong pizzeria in Washington. Mr. Alefantis and his employees have gotten caught up in a storm of fake news recently. A barrage of false articles on social media and fake news sites said their pizzeria was a front for a child-trafficking ring led by Hillary Clinton and her campaign chief, John D. Podesta.

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The stories — collected under the hashtag #pizzagate — have surprised Mr. Alefantis. He was a supporter of Mrs. Clinton’s and has prominent Democratic friends in Washington, but has never met the former Democratic presidential candidate, nor peddled children nor abused them.