Facebook is coming under fire because doctors are facing harassment on the platform by anti-vaccination activists.

The social network, which along with Google is being criticized for enabling anti-vaccination content to spread, was called out in The Guardian by a pro-vaccine naturopath named Elias Kass who was inundated with vitriol after he testified before a Washington state senate committee this month in favor of a bill that would eliminate personal and philosophical exemptions for childhood vaccinations and make them mandatory.

The doctor's Facebook page flooded with one-star reviews and he was reportedly called a "disgrace," a "pedophile" and a "Nazi pharma shill." According to the British news site, he disabled the page's reviews feature, but users were still able to write comments on posts, and Kass eventually put together a photo gallery containing hundreds of screenshots of abusive comments.

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"Their goal is to tell my patients what a bad person I am so I lose business," Kass told The Guardian. "It's made me reluctant to engage online."

This type of harassment is the latest manifestation of a movement of anti-vaxxers that has spread its message and grown partly because of Facebook's algorithms and policies.

The Guardian reports that the harassment of Kass appears to have been instigated in part by two anti-vaccine activists named Larry Cook and Erin Elizabeth who have built large followings on Facebook. Cook's Stop Mandatory Vaccination page has over 128,000 followers and features a range of sponsored posts purporting to show the problems with vaccination. There's also a Facebook group called Stop Mandatory Vaccinations with over 150,000 members. Elizabeth's Facebook page has over 500,000 followers and she runs a closed Facebook group called Holistic Lives Matter with about 53,000 members.

The World Health Organization calls anti-vaxxers a "top global threat," adding that "vaccine-hesitancy threatens to reverse progress in tackling vaccine-preventable diseases."

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Both Cook and Elizabeth posted links to Kass' Facebook page in their own groups, along with screenshots of his testimony, The Guardian reports.

Since the anti-vaccine groups are closed, the content posted in them tends to be one-sided and members are screened before they can join.

"It shows what a linchpin Facebook is," Kass told the Guardian. "That is their vehicle. Without the vehicle, who knows what they would be accomplishing."

When contacted by Fox News, the social network pointed to a range of changes it has made to tackle hate speech and disinformation, including the hiring of 30,000 across its safety and security team, and the use of machine learning and other tools to flag content that violates its policies.

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“We do not tolerate harassment on Facebook. We use a combination of AI, machine learning and manual review to proactively detect and remove harassment throughout our platform — including posts in groups and pages. And since we find that comments can be used for harm, we're working to give people the ability to hide or delete multiple comments at once," a Facebook spokesperson told Fox News via email.

Kass, who was honored by the Centers for Disease Control and Prevention for his work on childhood vaccination, told the British news that he's concerned about his ability to connect with parents now that he's been labeled as an "enemy" by these anti-vaccine groups.