Over the last decade, Dr. Mercola has built a vast operation to push natural health cures, disseminate anti-vaccination content and profit from all of it, said researchers who have studied his network. In 2017, he filed an affidavit claiming his net worth was “in excess of $100 million.”

And rather than directly stating online that vaccines don’t work, Dr. Mercola’s posts often ask pointed questions about their safety and discuss studies that other doctors have refuted. Facebook and Twitter have allowed some of his posts to remain up with caution labels, and the companies have struggled to create rules to pull down posts that have nuance.

“He has been given new life by social media, which he exploits skillfully and ruthlessly to bring people into his thrall,” said Imran Ahmed, director of the Center for Countering Digital Hate, which studies misinformation and hate speech. Its “Disinformation Dozen” report has been cited in congressional hearings and by the White House.

In an email, Dr. Mercola said it was “quite peculiar to me that I am named as the #1 superspreader of misinformation.” Some of his Facebook posts were only liked by hundreds of people, he said, so he didn’t understand “how the relatively small number of shares could possibly cause such calamity to Biden’s multibillion dollar vaccination campaign.”

The efforts against him are political, Dr. Mercola added, and he accused the White House of “illegal censorship by colluding with social media companies.”

He did not address whether his coronavirus claims were factual. “I am the lead author of a peer reviewed publication regarding vitamin D and the risk of Covid-19 and I have every right to inform the public by sharing my medical research,” he said. He did not identify the publication, and The Times was unable to verify his claim.

A native of Chicago, Dr. Mercola started a small private practice in 1985 in Schaumburg, Ill. In the 1990s, he began shifting to natural health medicine and opened his main website, Mercola.com, to share his treatments, cures and advice. The site urges people to “take control of your health.”