Dr. Daniel Stock, a physician in the state, made a slew of false claims about the coronavirus while testifying before a local school board in central Indiana this month. He declared that the recent increase in cases demonstrated that vaccines were ineffective, that people were better off with a cocktail of drugs and supplements to avoid hospitalization from the virus, and that masks did not help prevent infection spread.

His appearance has since become one of the most popular coronavirus misinformation videos on the internet. The videos have nearly 100 million likes and shares on Facebook, 6.2 million views on Twitter, at least 2.8 million views on YouTube, and more than 940,000 video views on Instagram.

The popularity of his talk highlights one of the pandemic’s more intriguing paradoxes. Even as many doctors work to save the lives of people infected with Covid-19, a small group of their medical colleagues has wielded enormous power in disseminating false and misleading information about the virus and vaccines.

There is a growing call among medical organizations to discipline physicians who spread false information. Last month, the Federation of State Medical Boards, which represents the organizations that license and discipline doctors, recommended that states consider taking action against doctors who share false medical claims, such as suspending or revoking medical licenses. According to the American Medical Association, spreading misinformation is a violation of the code of ethics that licensed doctors must follow.

Dr. Stock joined doctors like Dr. Joseph Mercola and Dr. Judy Mikovits, as well as a group called America’s Frontline Doctors, in generating large audiences for their bogus claims. According to public health officials, their and others’ statements have contributed to vaccine hesitancy and mask resistance, which has exacerbated the pandemic in the United States.

Some state medical boards have sanctioned doctors for their actions during the pandemic. The Oregon Medical Board ordered an emergency suspension of a doctor’s medical license in December after he violated a state order by not wearing a mask or requiring patients to wear masks. The decision prohibits the doctor from practicing medicine in Oregon until the governor lifts the state of emergency declared in response to the pandemic.

Dr. Chaudhry, on the other hand, said it was impossible to know how many states had launched investigations into doctors who spread misinformation. Such investigations are typically not made public until a decision is reached, which can take months.

Dr. Stock, 59, did not respond to multiple requests for comment for this story. He has been an Indiana licensed doctor since 1989, a year after graduating from Indiana University School of Medicine. According to his LinkedIn profile, he has worked in a number of hospitals, urgent care centers, and private practices throughout the state.

Dr. Stock distinguishes himself from conventional medicine on his website. “By presenting patients with all of their treatment options — whether that is a pill, a lifestyle change, therapy, or supplements — I help patients choose the option that works best for them,” according to the website. “This results in long-term healing rather than the short-term relief found in the traditional system.” On his website, he sells a variety of vitamins and supplements.

Dr. Stock is seen speaking at a Mt. Vernon Community School Corporation board meeting in Fortville, Indiana, in a video that has gone viral this month. He begins his statement, standing with his back to the camera and speaking at a rapid, nearly monotone clip, with the line, “Everything being recommended by the C.D.C. is actually contrary to the rules of science.” Then he cherry-picks academic studies to give the impression that widely held medical advice, such as wearing a mask and getting vaccinated, is ineffective.

YouTube, which prohibits videos that spread false information about the virus, stated that it would not remove the entire video of the meeting that the school board had posted online. “While we have clear policies in place to remove harmful Covid-19 misinformation, we also recognize the value of organizations like school boards using YouTube to share recordings of open public forums,” said Elena Hernandez, a YouTube spokeswoman.

Doctors spreading coronavirus misinformation “leverage the credibility of their titles and medical expertise to make their arguments appear more authoritative,” according to Rachel E. Moran, a University of Washington researcher who studies online misinformation, including about the Covid-19 vaccines.