Jeff Bezos, founder and CEO of Amazon, speaks with employees after he made a surprise appearance during a Amazon Veterans Day celebration, honoring the Warriors@Amazon, a group of employees who have served in the military and their spouses, in an event outside a hanger at the Long Beach Airport in Long Beach on Monday, November 12, 2018.

"There is strong evidence to suggest that at least part of the source of this trend is the degree to which medically inaccurate information about vaccines surface on the websites where many Americans get their information, among them Amazon," Schiff wrote. "As the largest online marketplace in the world, Amazon is in a unique position to shape consumption.

Congressman Adam Schiff , D-Calif., sent a letter to Amazon CEO Jeff Bezos on Friday, expressing concern about the emerging anti-vaccination trend related to the spread of misinformation on the internet, calling it a "direct threat to public health." Schiff cited a World Health Organization report that called "vaccine hesitancy" a top health threat in 2019.

Amazon isn't the only company that's under pressure to act. Pinterest is taking steps to crack down on health misinformation by blocking content related to searches about vaccines. YouTube announced it would not allow users to make money from anti-vaccine videos with ads, and Facebook said articles deemed false would rank lower in feeds.

Earlier this month, Schiff sent an open letter to both Facebook and Google to express similar concerns, and both companies said they would submit a response. CNN recently found anti-vaccination books for sale on Amazon, with titles like "We Don't Vaccinate!" and "Shoot 'Em Up: The Truth About Vaccines," as well as similar content available to stream through Amazon Prime.

Schiff said it's up to Amazon and the other major platforms to properly police the content on their sites. He asked Bezos if inaccurate medical information about vaccines violates Amazon's terms of service, what steps the company is taking to address the problem and whether it accepts paid advertising from "anti-vaccine activists and groups."

"As a Member of Congress who is deeply concerned about declining vaccination rates around the nation, I am requesting additional information on the steps that you currently take to provide medically accurate information on vaccinations to your users, and to encourage you to consider additional steps you can take to address this growing problem," Schiff wrote.

Amazon didn't immediately respond to a request for comment.

WATCH: Pinterest eliminates search results for vaccination posts