As a disturbing number of measles outbreaks crop up around the United States, Facebook is facing challenges combating widespread misinformation about vaccinations on its platform, which has become a haven for the anti-vaccination movement.

The World Health Organization recently named “vaccine hesitancy” as one of the biggest global health threats of 2019. But on Facebook, in public pages and private groups with tens of thousands of members, false information about vaccines — largely stemming from a now-debunked 1998 study that tied immunizations to autism — is rampant and tough to pin down. In the bubble of closed groups, users warn against the dangers of vaccinations, citing pseudoscience and conspiracy theories.

Facebook has publicly declared that fighting misinformation is one of its top priorities. But when it comes to policing misleading content about vaccinations, the site faces a thorny challenge. The bulk of anti-vaccination content doesn’t violate Facebook’s community guidelines for inciting “real-world harm,” according to a spokesperson, and the site’s algorithms often promote unscientific pages or posts about the issue. Parents are left to wade through the mire, and as the viral spread of fake news has shown, many users have trouble distinguishing between reliable sources and unreliable ones.

The rise of “anti-vaxx” Facebook groups is overlapping with a resurgence of measles.

Wendy Sue Swanson, a pediatrician at Seattle Children’s Hospital and spokeswoman for the American Academy of Pediatrics, recently met with Facebook strategists about dealing with public health issues, including misinformation about vaccines, on the platform. Swanson said that it’s not Facebook’s job to police the dialogue around immunizations, but rather to make sure users have ample access to scientifically valid content.

“You wouldn’t go see a pediatrician who doesn’t hold medical certification, but on the internet, you might listen to them,” Swanson said. “Facebook isn’t responsible for changing quacks but they do have an opportunity to change the way information is served up.”