John Podesta after addressing the Democratic National Convention in Philadelphia. Drew Angerer/Getty Images SAN FRANCISCO — Fake news is "undermining our democracy," but it's something people may not notice if they get their news from the mainstream media, former Clinton campaign chair John Podesta warned on Wednesday.

"It’s sinister, and it’s a vile stream that runs under the ground and it begins to have an effect and takes its toll," Podesta said on-stage at the NewCo Shift Forum.

The "subterranean" power of fake news was something Podesta experienced first hand when he and Hillary Clinton were accused of running a child sex ring inside an upscale D.C. pizzeria called Comet Ping Pong. It was a fake news story that was debunked immediately in the mainstream news, but that didn't stop one North Carolina man from entering the restaurant with a gun to investigate it for himself. Podesta said it was a "horrifying episode" for the employees, and the people in the community, but it's hard to pull the claims back once they've been seeded online.

"All that stuff exists, you don’t see it, you don’t feel it if you’re largely living in reality in a mainstream media," Podesta said. (Since the election, the Trump administration has also used the term "fake news" to attack the mainstream media, especially CNN.)

Podesta pointed to a BuzzFeed news investigation that found fake news articles were more widely shared and liked than the top political articles from the election. Part of the viral spread, he believes, was also foreign governments spinning up bot farms.

Since the election, companies like Facebook and Google have pledged to crack down on the spread of fake news. This week, Facebook said it would partner with leading French media outlets to tackle the fake news spreading on its platform ahead of the elections in France, which will happen in April and May.

Podesta applauded media companies for beginning to acknowledge their responsibility — something Mark Zuckerberg hesitated to do at first after the election — but said it's still a serious threat that needs to be addressed.

"It's undermining our democracy," Podesta said. "I don’t know what technical fix I’m asking for, but I think the companies need to take it seriously."