In 2016, doctors discovered that Joshua Nerius, a 30-year-old software product manager in Chicago, had developed measles. After texting with his mom from the emergency room, he found out that he had never been vaccinated as a child.

“It makes me so angry. My parents thought they were doing the right thing. They were persuaded by the anti-vaxxers,” he told CNN Thursday.

Nerius said that he doesn’t blame his parents for subscribing to anti-vax views because at the time there weren’t ample ways to read about the science behind vaccines. But now, he said that “it really frustrates me” to see parents refusing to vaccinate their children due to misinformation on vaccines online, which has contributed to a string of recent measles outbreaks.

“The science on this has been settled. It’s been solved. When I look at where we are today, with people who are willfully deciding to ignore the facts, it really frustrates me,” Nerius said. “I just don’t understand the mindset of people who want to spread fear.”

In recent years, anti-vaxxers have used social media to spread misinformation about vaccines, including false claims that vaccines cause autism.

Public health officials have warned that the misinformation and anti-vax parents’ failure to immunize their children have contributed to recent measles outbreaks, and worry that they are getting harder to contain.

In the state of Washington, officials declared a state of emergency in January. Most of the reported measles cases were in unvaccinated children.

Last month, 18-year-old Ethan Lindenberger made headlines for defying his anti-vax mother’s wishes by getting vaccinated.

During congressional testimony about the issue earlier this week, Lindenberger told lawmakers that his mother’s source of information was “mainly Facebook.”