But over the past decade a single donor has contributed more than $2.9 million to the National Vaccine Information Center, accounting for about 40 percent of the organization’s funding, according to the most recent available tax records. That donor, osteopathic physician Joseph Mercola, has amassed a fortune selling natural health products, court records show, including vitamin supplements, some of which he claims are alternatives to vaccines.

In recent years, the center has been at the forefront of a movement that has led some parents to forgo or delay immunizing their children against vaccine-preventable diseases such as measles. Health officials say falling vaccination rates contributed to the infectious virus sickening more than 1,200 people in the United States this year, the largest number in more than 25 years. Measles outbreaks are surging worldwide, including in Samoa – where nearly 80 people have died since mid-October, the great majority of them young children and infants.

The Northern Virginia-based National Vaccine Information Center lists Mercola.com as a partner on its homepage and links to the website, where readers can learn about and purchase Mercola’s merchandise.

Last month, Mercola wrote on his website that measles “continues to be a Trojan Horse for increasing vaccine mandates.” A page that was recently removed said that “vitamin C supplementation is a viable option for measles prevention.” Elsewhere on the site, a page about vitamin D includes the headline, “Avoid Flu Shots With the One Vitamin that Will Stop Flu in Its Tracks.”

Mercola, whose claims about other products have drawn warnings from regulators, has also given at least $4 million to several groups that echo the anti-vaccine message. His net worth, derived largely from his network of private companies, has grown to “in excess of $100 million,” he said in a 2017 affidavit.

Mercola said in emails to The Washington Post that he contributes to the center because he believes in its mission. He said he offers “simple, inexpensive and safe alternatives to the conventional medical system, which is contributing to the premature death of millions and is causing needless pain and suffering in great part because multinational corporations want to increase their revenues.”

He declined to be interviewed and did not respond to questions about how much profit his vitamin D and C supplements generate relative to the rest of his wide-ranging merchandise, which includes organic cotton underwear and pet food. Supplements containing those vitamins are among Mercola’s “top products,” his website says.

In a statement, his media team said the claims on Mercola’s website relate to vitamin D and vitamin C generally and “do not mention Dr. Mercola’s products whatsoever.”

Earlier this month, Samoan anti-vaccine activist Edwin Tamasese, who touted vitamins as an alternative to vaccination, was arrested for allegedly claiming on social media that measles vaccinations would result in mass deaths.

“The anti-vaxxers unfortunately have been slowing us down,” government spokesman Afamasaga Rico Tupai said in an interview with a New Zealand television station after the arrest. “We’ve had children who have passed away after coming to the hospital as a last resort and then we find out the anti-vaccine message has got to their families and that’s why they’ve kept these kids at home.”

Vaccination rates plummeted on the tiny Pacific island in the past year after two infants died within hours of receiving the measles, mumps and rubella vaccine – fatalities that were highlighted by anti-vaccination activists. Two Samoan nurses were later convicted of manslaughter for improperly mixing the vaccine with a muscle relaxant instead of water.

The National Vaccine Information Center was founded in 1982 by Barbara Loe Fisher, who has said that her son was injured by a vaccine. The group claimed credit this year for helping to defeat legislation in a dozen states that would have made it harder for parents to opt out of vaccinating their children.

At the beginning of last year’s flu season, Fisher and Mercola appeared in a YouTube video urging people to be skeptical about flu shots. Mercola claimed that vaccines have been associated with “deaths and permanent neurological complications,” and he said vitamin D supplements were among “far more effective, less expensive and less risky alternatives.”

Such claims are highly misleading, government health officials say. For example, they say, while on very rare occasions people have developed the neurological disorder Guillain-Barré syndrome after receiving the flu shot, research suggests the disorder is more strongly associated with contracting the flu itself than with receiving the vaccine. In addition, while some studies have suggested that vitamin D might help prevent the flu, others have found no such benefit, and the Centers for Disease Control and Prevention have said that the best way to protect against the infection is vaccination.

Mercola referred The Post to materials he said showed evidence that vaccines can be harmful, including some studies on vaccines no longer in use. Experts and government health officials say medical evidence overwhelmingly supports the safety and efficacy of vaccines.

“He mixes the boring, sensible health advice with pseudoscientific advice in such a way that it’s hard for someone without a medical background to figure out which is which,” said David Gorski, an oncologist and surgeon at Wayne State University who is widely regarded as a leading expert on the anti-vaccine movement.

Fisher said in an interview that Mercola has asked for nothing in exchange for his donations and that the National Vaccine Information Center does not sell or advertise Mercola’s products on its site.

“I do not take funding for a quid pro quo,” she said. “When [Mercola] called me, he said, ‘I admire your work. I’d like to help you.’ ”

The center’s homepage, which the group says was visited more than 1.2 million times last year, displays Mercola.com’s logo. An affiliated website run by Fisher’s group refers numerous times to Mercola.com as one of the most popular health and wellness websites.

Asked if his companies benefit from his donations to the anti-vaccine group, Mercola said in an email that “being an adversary to powerful industries is not a positive for a business like mine.”

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Mercola, 65, started his website in 1997. He had been seeing patients for more than a decade as a licensed osteopath – a practitioner in a field of medicine that often uses joint manipulation to treat illness – and had became convinced that conventional medicine was part of the problem, not the solution, he said in an email.

Along with giving health advice, the website advertised services offered at Mercola’s medical clinic outside Chicago. Among them was a months-long program to help patients “heal mild to serious disease, lose weight” and “permanently abandon” addictions using a “natural-centered approach” for an average price of $2,000, the website said.

By the mid-2000s, Mercola’s focus was shifting to selling products. “I didn’t want to advertise products and businesses that I didn’t trust or believe in – so I formulated, tested, and sold my own brand of products to support the website,” he said in an email.

Susan Woller, then Mercola’s director of business development, described him in an interview as an “excellent marketer” and “voracious learner” who developed ideas on everything from new protein bars to safer cookware. She said his profit margin increased dramatically as he shifted to marketing products under his own brand. She declined to say by how much.

“He is doing all the research and delivering information to his readers,” Woller said. “When you do that and you marry that with a product that you’ve private-labeled, and people respond to that, you can’t help but make some money off it.”

But federal regulators had concerns.

In 2005 and again in 2006, the Food and Drug Administration warned Mercola that he was illegally claiming some of his products, including coconut oil, could help prevent or treat heart disease, cancer and other illnesses. The agency told him that many such products could be marketed as “dietary supplements” if claims about treating or preventing disease were removed. His website now includes the statement: “These products are not intended to diagnose, treat, cure, or prevent any disease.”

Mercola said he stopped seeing patients in 2009. By the following year, the businesses were generating $3 million a month, according to records that surfaced in an unrelated lawsuit between Mercola and his financial advisers. He later shifted his businesses to an office building in Cape Coral, Fla.

In 2011, Mercola received another warning from the FDA, this time regarding his claims about thermography, a procedure in which an infrared camera detects patterns of heat and blood flow in the body. The agency wrote that he inaccurately claimed thermography was more sensitive than mammography in detecting diseases such as breast cancer and threatened to impose fines or take other action if those statements were not rescinded.

Mercola has since stopped offering thermography, according to former employees and business records. He said in an email that the warning was based on “political pressure.”

The FDA has taken no enforcement action against Mercola over his vaccine claims. The agency said in a statement that it has no role in approving dietary supplements or in evaluating claims about them – unless those products are purported to treat, diagnose, prevent or cure diseases and are reported to the agency as unapproved drugs. Agency officials did not answer questions about Mercola’s claims regarding vitamins C and D.

In 2016, in response to a complaint from the Federal Trade Commission, Mercola refunded nearly $2.6 million to more than 1,300 people who bought tanning beds that he claimed could reduce the risk of skin cancer. The FTC said the claims constituted a “deceptive act,” pointing out that the product could actually increase the risk of skin cancer.

“Defendants have been unjustly enriched as a result of their unlawful acts or practices,” the agency wrote in the complaint, which was settled when Mercola agreed to pay the refunds.

He stopped selling the tanning beds. Even so, he stood by his claims about their benefits and said in an email that the FTC’s action “was based on criticisms and political pressure” from dermatologists and public health officials, who he said were “wrong.”

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Mercola and Fisher connected about a decade ago, they recalled in a recent video posted to his YouTube channel. In the video, Mercola said the National Vaccine Information Center was then “in transition and was almost ready to shut down.”

“That’s actually true,” Fisher responded.” You made a very big difference at a critical point in our history, and I will always be grateful.”

At the time, a key study that alleged a link between vaccines and autism was unraveling. The research paper, by British gastroenterologist Andrew Wakefield, was eventually retracted by prestigious medical journal the Lancet, and Wakefield was banned from practicing medicine in Britain.

The anti-vaccine movement began pivoting to a broader message blaming vaccines for numerous illnesses. That aligned with growing interest in alternative medicine and increasing skepticism about the government’s role in parental decision-making.

The resurgent movement found a handful of wealthy patrons, including hedge fund manager and philanthropist Bernard Selz and his wife, Lisa. The Selzes gave $200,000 to a legal fund for Wakefield in 2012, and they went on to give more than $3 million to anti-vaccine groups, including one that held forums this year in Brooklyn, the epicenter of the measles outbreak, The Post previously reported.

The $2.98 million Mercola has given to Fisher’s group since 2009 came from the Natural Health Research Foundation, a private foundation that is entirely funded by his business and that he leads as president, tax records show.

The foundation has also given more than $3.3 million to the Organic Consumers Association, a health and environmental advocacy group, tax records show. On its website, the organization advertises Mercola’s products and reposts some of his content about vaccines and other topics, as well as articles by prominent anti-vaccine activist Robert F. Kennedy Jr.

The group’s associate director, Katherine Paul, said that the website’s content is not linked to Mercola’s financial support, and that the organization had posted Mercola’s articles before he started contributing.

Mercola said he supports the Organic Consumers Association because of the group’s dedication to improving food and agriculture.

While Fisher emphasized that she does not promote any retailers’ products, she told potential sponsors of an anti-vaccine conference her group held in 2009 that the event could help them gain “access to both national and international markets” and “increase sales and visibility,” promotional material shows.

Mercola attended the conference and received an “NVIC visionary award.” Mercola did not respond to questions about whether he was a sponsor. He donated $20,000 that year to the National Vaccine Information Center through his foundation. Without elaborating, Fisher said the money “was not an earmarked donation.”

The following year, Fisher’s group and Mercola.com launched a website that tracks vaccine-related legislation in every state. The site provides activists with detailed information, including how to sign up for public comment to support or oppose legislation in their state, where to park to attend a public hearing and what color T-shirt to wear to rallies.

In 2011, Mercola co-sponsored a 15-second video ad on a Jumbotron in Times Square. The ad featured an image of a woman holding a baby with the message “Vaccines: Know the Risks.” Logos of both the National Vaccine Information Center and Mercola.com were prominently displayed.

The ad prompted a heated letter from the American Academy of Pediatrics to CBS Outdoor, then the owner of the Jumbotron. “By providing advertising space to an organization like the NVIC . . . you are putting the lives of children at risk, leaving them unprotected from vaccine-preventable diseases,” wrote O. Marion Burton, then the president of the medical group.

According to Fisher, most of the National Vaccine Information Center’s financial support comes from “individual donations, small foundation grants under $5,000 and in-kind donations.”

In addition to the large contributions from Mercola, Fisher’s group has received financial support from Focus for Health. That foundation, funded by wealthy New Jersey businessman Barry Segal, gave Fisher’s group more than $400,000 from 2011 to 2017, according to tax records.

In September of this year, as flu season approached, Mercola and the National Vaccine Information Center co-sponsored their 10th annual “vaccine awareness week.” Mercola offered to match all donations made to the center that week up to $100,000, according to a video posted to his YouTube channel.

In a video posted the following day, Fisher described her group’s recent successes in state legislatures across the country. Those states include Colorado, Minnesota, Oregon and Texas, Fisher said.

“I look forward to another 30 years of working with you,” she told Mercola.