The rise of Robert F. Kennedy Jr. from fringe figure to the prospective head of U.S. health policy was fueled by skepticism and distrust of the medical establishment — views that went viral in the Covid-19 pandemic.

People once dismissed for their disbelief in conventional medicine are now celebrating a new champion in Washington. Scientists, meanwhile, are trying to figure how they could have managed the pandemic without setting off a populist movement they say threatens longstanding public-health measures.