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Facebook, Pinterest, YouTube, and Amazon are all working to keep dangerous anti-vaccination rumors and hoaxes from getting attention.

Twitter’s Jack Dorsey, unintentionally it seems, is doing the opposite.

Dorsey, who has been on a media tear lately, sitting for interviews with lots of podcasters, tweeted on Tuesday that he had a “great conversation” with author Ben Greenfield on Greenfield’s podcast. “[I] appreciate all you do to simplify the mountain of research focused on increasing one’s healthspan! Grateful for you,” Dorsey wrote.

The problem is that Greenfield, who has written books about health and triathlons, also promotes anti-vaccination hoaxes. “Vaccines do indeed cause autism,” he tweeted in early February, adding that people shouldn’t trust the widely respected fact-checking service, Snopes, to get their news.

The myth that vaccinations cause autism has been refuted by the world’s top health organizations, and poses a real danger to children whose parent’s choose not to vaccinate them — a measles outbreak is currently spreading in several states due to lax vaccination policies. Tweets promoting anti-vaccination myths are not against Twitter’s rules, but those promoting those myths or hoaxes as an advertisement would be against Twitter’s guidelines.

A Twitter spokesperson tells Recode that Dorsey wasn’t aware of Greenfield’s take on vaccines, and neither was the company. The topic apparently didn’t come up during the podcast.

But the optics are not good, and Dorsey’s tweet praising Greenfield for his health expertise is still up and viewable to Dorsey’s 4.15 million followers, almost four hours after it was first posted.

It’s believable that Dorsey (or his public relations team) wasn’t aware of his host’s spurious medical advice, though Greenfield’s tweet is less than two months old. Endorsements from figures like Dorsey carry a lot of weight with the general public, which is why many found it concerning that the Twitter CEO praised Greenfield as a health expert given his take on vaccines.

Anti-vax myths aside, though, what’s incredible is that Dorsey continues to accidentally step in it.

In early December, Dorsey posted glowingly about a recent trip he made to Myanmar to meditate, ignoring the fact that Myanmar is a country where social media platforms like Facebook have helped spur an attempted genocide. He was roasted for failing to acknowledge the country’s problems while encouraging people to go visit.

In November, Dorsey also made headlines when he posed with a sign that read “Smash Brahminical Patriarchy,” a reference to India’s caste system. Twitter, again, claimed Dorsey was ignorant of the sign’s meaning. (Dorsey also said last month that he thinks Tesla’s Elon Musk is great at Twitter. Musk, of course, is in trouble with the SEC because of his tweets.)

Even though anti-vax myths aren’t against Twitter’s terms of service, Dorsey is indeed trying to clean up the platform. “Health” has been the company’s unofficial catchphrase for the past year, and Twitter is slowly trying to measure how healthy user interactions are on the service in an effort to clean up the platform.

Coincidentally, about three hours after Dorsey’s tweet, Amazon announced that it would remove books about unproven “autism cures” from its website.

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